Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2018	Mar. 01, 2019
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	ARCT
Entity Registrant Name	Arcturus Therapeutics Ltd.
Entity Central Index Key	0001566049
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		10,761,523
Entity Public Float		$ 41.3

CONSOLIDATED BALANCE SHEETS - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 36,709,000	$ 24,965,000
Restricted cash		166,000
Short-term investments		23,608,000
Accounts receivable, net	4,481,000	480,000
Prepaid expenses and other current assets	638,000	1,059,000
Intangible asset held for sale		590,000
Total current assets	41,828,000	50,868,000
Property and equipment, net	1,975,000	1,049,000
Equity method investment	288,000
Non-current restricted cash	107,000	107,000
Total assets	44,198,000	52,024,000
Current liabilities:
Accounts payable	2,398,000	1,790,000
Accrued liabilities	3,907,000	2,793,000
Deferred revenue	6,272,000	6,457,000
Total current liabilities	12,577,000	11,040,000
Deferred revenue, net of current portion	7,534,000	7,190,000
Long-term debt	9,911,000
Deferred rent	534,000
Total liabilities	30,556,000	18,230,000
Commitments and contingencies (Note 13)
Shareholders’ equity:
Ordinary Shares: 30,000 shares authorized, 10,762 issued, 10,719 outstanding and 43 held in treasury at December 31, 2018; NIS 0.07 par value; 30,000 shares authorized, 10,699 issued, 10,656 outstanding and 43 held in treasury at December 31, 2017;	214,000	212,000
Additional paid-in capital	58,302,000	56,674,000
Accumulated other comprehensive loss		(3,000)
Accumulated deficit	(44,874,000)	(23,089,000)
Total shareholders’ equity	13,642,000	33,794,000
Total liabilities and shareholders’ equity	$ 44,198,000	$ 52,024,000

CONSOLIDATED BALANCE SHEETS (Parenthetical) - ₪ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Common stock, par value		₪ 0.07
Common stock, shares authorized	30,000,000	30,000,000
Treasury shares	43,000	43,000
Common stock, shares issued	10,762,000	10,699,000
Common stock, shares outstanding	10,719,000	10,656,000

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Collaboration revenue	$ 15,753	$ 12,998
Operating expenses:
Research and development, net	16,982	15,918
General and administrative	20,582	7,572
Total operating expenses	37,564	23,490
Net loss from operations	(21,811)	(10,492)
Loss from equity method investment	(302)
Finance income (expense), net	328	(409)
Net loss before taxes	(21,785)	(10,901)
Income tax expense		(1)
Net loss	$ (21,785)	$ (10,902)
Net loss per share, basic and diluted	$ (2.16)	$ (3.53)
Weighted-average shares outstanding, basic and diluted	10,069	3,087
Comprehensive loss:
Net loss	$ (21,785)	$ (10,902)
Unrealized gain (loss) on short-term investments	3	(3)
Comprehensive loss	$ (21,782)	$ (10,905)

STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Series Seed Preferred Stock [Member]	Series A Preferred Stock	Ordinary Shares [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2016	$ 1,577				$ 13,764		$ (12,187)
Balance (in shares) at Dec. 31, 2016		1,284,000	1,481,000	2,801,000
Net loss	(10,902)						(10,902)
Unrealized gain (loss) on short-term investments	(3)					$ (3)
Share-based compensation	2,170				2,170
Issuance of common shares upon exercise of share options	675				675
Issuance of common shares upon exercise of share options (in shares)				348,000
Issuance of shares upon exercise of warrants	160				160
Issuance of shares upon exercise of warrants (in shares)				189,000
Issuance of shares upon conversion of notes	5,957				5,957
Issuance of shares upon conversion of notes (in shares)				617,000
Conversion of Preferred Shares to Ordinary Shares (in shares)		(1,284,000)	(1,481,000)	2,765,000
Beneficial conversion expense from notes	348				348
Issuance of shares in connection with merger, net	33,812			$ 212	33,600
Issuance of shares in connection with merger, net (in shares)				3,979,000
Balance at Dec. 31, 2017	33,794			$ 212	56,674	(3)	(23,089)
Balance (in shares) at Dec. 31, 2017				10,699,000
Net loss	(21,785)						(21,785)
Unrealized gain (loss) on short-term investments	3					$ 3
Share-based compensation	1,259				1,259
Issuance of common shares upon exercise of share options	$ 371			$ 2	369
Issuance of common shares upon exercise of share options (in shares)	62,224			63,000
Balance at Dec. 31, 2018	$ 13,642			$ 214	$ 58,302		$ (44,874)
Balance (in shares) at Dec. 31, 2018				10,762,000

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
OPERATING ACTIVITIES:
Net loss	$ (21,785)	$ (10,902)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	582	410
Share-based compensation expense	1,259	2,170
Loss from equity method investment	302
Non cash interest expenses	38	150
Debt conversion expense		348
Changes in operating assets and liabilities:
Accounts receivable	(4,001)	3,153
Prepaid expenses and other assets	421	(202)
Accounts payable	578	(1,537)
Accrued liabilities	1,687	(104)
Deferred revenue	159	6,054
Net cash used in operating activities	(20,760)	(460)
INVESTING ACTIVITIES:
Acquisition of property and equipment	(1,478)	(251)
Purchases of short-term investments	(6,594)
Proceeds from sale of equipment		29
Proceeds from maturities of short-term investments	30,206	10,577
Net cash provided by investing activities	22,134	10,355
FINANCING ACTIVITIES:
Proceeds from long-term debt, net of debt issuance costs	9,872
Proceeds from exercise of stock options	332	711
Proceeds from issuance of convertible promissory notes		5,650
Proceeds from exercise of warrants		160
Net cash received in the issuance of shares for the net assets of Alcobra Ltd.		477
Net cash provided by financing activities	10,204	6,998
NET INCREASE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	11,578	16,893
Cash, cash equivalents and restricted cash at beginning of year	25,238	8,345
Cash, cash equivalents and restricted cash at end of year	36,816	25,238
Supplemental disclosure of cash flow information:
Cash paid for interest	146
Cash paid for income taxes		35
Sale of intangible assets for equity method investment	590
Purchase of property and equipment in accounts payable	30
Release of repurchase liability for restricted shares	$ 39
Conversion of notes to ordinary shares		5,957
Fair value of assets acquired, excluding cash, cash equivalents and restricted cash		35,241
Less: liabilities assumed		(1,906)
Net assets acquired, excluding cash, cash equivalents and restricted cash		$ 33,335

Organization

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization

Note 1. Organization

Description of Business

Arcturus Therapeutics Ltd. and its subsidiaries (referred to as the “Company”) is a RNA medicines company focused on significant opportunities in rare, liver, and respiratory diseases. The Company’s key proprietary technology has the potential to address the major hurdles in RNA development, namely the effective and safe delivery of RNA therapeutics to disease-relevant target tissues.

Reverse Merger

On November 15, 2017, Alcobra Ltd. acquired Arcturus Therapeutics, Inc. pursuant to a merger between the companies (the “merger”). Prior to the merger, Alcobra Ltd.’s net assets consisted of cash, investments and nominal non-operating assets. Upon consummation of the merger, Alcobra Ltd. adopted the business plan of Arcturus Therapeutics, Inc. In connection with the merger, Alcobra Ltd. agreed to acquire all of the outstanding common stock of Arcturus Therapeutics, Inc. in exchange for the issuance of an aggregate 6,631,712 of Alcobra Ltd.’s Ordinary Shares, par value 0.07 NIS per share (the “Ordinary Shares”), after giving effect to a 1-for-7 reverse split effected immediately prior to the merger. As a result of the merger, Arcturus Therapeutics, Inc. became a wholly-owned subsidiary of Alcobra Ltd. While Alcobra Ltd. was the legal acquirer in the transaction, Arcturus Therapeutics, Inc. was deemed the accounting acquirer. Immediately after giving effect to the merger, on November 15, 2017, Alcobra Ltd. changed its name to Arcturus Therapeutics Ltd. (“Arcturus” or the “Company”). On November 16, 2017, the Company commenced trading under the symbol “ARCT.” The Company’s principal executive offices and location of all operations are in San Diego, California.

In accordance with the authoritative literature, a transaction where a private company merges into a public company with no operations and nominal net assets should be accounted for as a capital transaction rather than a business combination. Consequently, the reverse merger was accounted for as an issuance of shares by the Company for the net assets of Alcobra Ltd., accompanied by a recapitalization. Excess of considerations paid over net assets acquired and other merger-related costs were recorded as a charge to additional paid-in capital as discussed in Note 6. While Alcobra Ltd. was the legal acquirer in the merger, Arcturus was deemed the accounting acquirer. As a result, the financial statements of the Company prior to the merger date are the historical financial statements of Arcturus whereas the financial statements of the Company after the merger date reflect the results of the operations of Arcturus and Alcobra Ltd. on a combined basis. All historical information presented herein has been retroactively restated to reflect the effect of the merger shares exchange ratio, reverse stock split and change to the authorized number of Ordinary Shares in accordance with Accounting Standards Codification Topic 260, “Earnings Per Share”.

Going Concern

The Company’s activities since inception have consisted principally of performing research and development activities and raising capital. The Company’s activities are subject to significant risks and uncertainties, including failing to secure additional funding before the Company achieves sustainable revenues and profit from operations.

The Company is a pre-clinical bioscience company that is dependent on obtaining external equity and debt financings to fund its operations. Historically, the Company’s primary source of financing has been through the sale of its securities, through issuance of debt and through collaboration agreements. The Company raised $10.0 million in gross proceeds from a long-term debt agreement executed in October 2018 (Note 9). In addition, in October 2018, the Company entered into a Sales Agreement (the “Sales Agreement”) with Leerink Partners LLC (“Leerink”), pursuant to which it may sell from time to time, at its option, up to an aggregate of $30.0 million of the Company’s Ordinary Shares through Leerink, as sales agent. Research and development activities have required significant capital investment since the Company’s inception.

The Company expects its operations to continue to require cash investment to pursue the Company’s research and development activities, including preclinical studies, formulation development, clinical trials and related drug manufacturing. The Company has experienced net losses since its inception and as of December 31, 2018 has an accumulated deficit of $44.9 million. The Company expects to continue to incur additional losses for the foreseeable future, and the Company will need to raise additional debt or equity financing or enter into additional collaborations to fund its development. The ability of the Company to transition to profitability is dependent on identifying and developing successful mRNA drug candidates. In the near future, if the Company is not able to achieve planned milestones, incurs costs in excess of its forecasts, or does not meet covenant requirements of its debt (Note 9), it will need to reduce discretionary spending, discontinue the development of some or all of its products, which will delay part of its development programs, all of which will have a material adverse effect on the Company’s ability to achieve its intended business objectives. There can be no assurances that additional financing will be secured or, if secured, will be on favorable terms. Management has prepared cash flow forecasts which indicate that based on the Company’s expected operating losses and negative cash flows, there is substantial doubt about the Company’s ability to continue as a going concern within twelve months after the date that the financial statements for the year ended December 31, 2018, are issued. The accompanying consolidated financial statements have been prepared assuming that the Company will continue as a going concern, which contemplates the realization of assets and the settlement of liabilities and commitments in the normal course of business. The consolidated financial statements do not reflect any adjustments related to the recoverability and classification of assets or the amounts and classification of liabilities that might be necessary if the Company is unable to continue as a going concern.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Note 2. Summary of Significant Accounting Policies

Basis of Presentation and Principles of Consolidation

The accompanying consolidated financial statements include the accounts of Arcturus Therapeutics Ltd. and its subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation. These financial statements are prepared in conformity with accounting principles generally accepted in the United States (U.S. GAAP), which requires management to make estimates and assumptions regarding the valuation of certain debt and equity instruments, the equity method investment, share-based compensation, accruals for liabilities, income taxes, revenue and deferred revenue, expense accruals, and other matters that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could materially differ from those estimates.

Segment Information

Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. The Company and its chief operating decision-maker view the Company’s operations and manage its business in one operating segment which is the research and development of medical applications for the Company’s nucleic acid-focused technology.

Cash and Cash Equivalents

Cash equivalents are short-term highly liquid investments that are readily convertible to cash with original maturities of three months or less at the date of purchase.

Restricted cash

Restricted cash represents cash required to be set aside as security for lease payments and to maintain a letter of credit for the benefit of the landlord for the Company’s offices. At December 31, 2018 and 2017, the Company had restricted cash of $107,000 in conjunction with property leases in San Diego, California, and such restriction is expected to be removed at the end of the lease term in 2025. At December 31, 2017, the Company also had restricted cash of $166,000 in conjunction with property leases in Israel, and such restriction was lifted in 2018.

Short-term Bank Deposits

Short-term bank deposits (Note 4) are deposits with maturities of more than three months and up to one year when acquired. Short-term bank deposits are presented at their cost, including accrued interest and are included in the balance of short-term investments in the consolidated balance sheet.

Short-term Investments

The Company accounts for short-term investments (Note 4) in accordance with ASC No. 320, Investments- Debt and Equity Securities. Management determines the appropriate classification of its investments at the time of purchase and reevaluates such determinations at each balance sheet date.

The Company has classified all of its debt securities and certificates of deposit as available-for-sale securities. Available-for-sale securities are carried at fair value, with the unrealized gains and losses reported in accumulated other comprehensive loss in shareholders’ equity. Realized gains and losses on sales of investments are included in interest income and are derived using the specific identification method for determining the cost of securities.

The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization together with interest and dividends on securities are included in interest income.

The Company recognizes an impairment charge when a decline in the fair value of its investments in debt securities below the amortized cost basis of such securities is judged to be other-than-temporarily impaired. Factors considered in making such a determination include the duration and severity of the impairment, the reason for the decline in value, the potential recovery period and if the entity has the intent to sell the security, or if it is more likely than not that it will be required to sell the security before recovery of its amortized cost basis. The Company did not recognize any other-than-temporary impairment charges on its marketable securities during the year ended December 31, 2017. The Company held no available-for-sale securities as of December 31, 2018.

Fair Value Measurements

Fair value is defined as the exit price, or the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants as of the measurement date. A hierarchy has been established for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the most observable inputs be used when available (Note 5).

Observable inputs are inputs market participants would use in valuing the asset or liability developed based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the factors market participants would use in valuing the asset or liability developed based upon the best information available under the circumstances. The hierarchy consists of three levels. Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities. Level 2 inputs include quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, and inputs (other than quoted prices) that are observable for the asset or liability, either directly or indirectly. Level 3 inputs are unobservable inputs for the asset or liability. Categorization within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement.

Accounts Receivable

Accounts receivable are recorded at the net invoice value and are non-interest bearing. The Company considers receivables past due based on the contractual payment terms. The Company reserves specific receivables if collectability is no longer reasonably assured. Estimates for allowances for doubtful accounts are determined based on existing contractual obligations, historical payment patterns, and individual customer circumstances. The Company reevaluates such reserves on a regular basis and adjusts its reserves as needed. Once a receivable is deemed to be uncollectible, such balance is charged against the reserve. No reserves have been recorded as of December 31, 2018 or 2017.

Concentration of Credit Risk and Significant Customers

Financial instruments, which potentially subject the Company to concentrations of credit risk, consist of cash, cash equivalents and short-term investments. The Company limits its exposure to credit loss by placing its cash, cash equivalents, and investments with high credit quality financial institutions in instruments with short maturities.

There was one customer that comprised 96% of the total accounts receivable balance at December 31, 2018 and one customer that comprised the total accounts receivable balance at December 31, 2017.

For the year ended December 31, 2018, the Company’s top three customers collectively represented 80% of the Company’s total revenue. For the year ended December 31, 2017, there were three customers that collectively represented 92% of the Company’s total revenue.

Intangible Assets Held for Sale and Equity Method Investment

At the end of the second quarter of 2018, the Company completed the sale of its intangible assets related to the ADAIR technology, which was accounted for as an intangible asset held for sale as of December 31, 2017. Pursuant to the asset purchase agreement for ADAIR, the Company received a 30% ownership interest in the common stock of a privately held company in consideration for the sale of the ADAIR technology. As this ownership interest is greater than 20% and one executive of the Company holds a seat on the investee’s board of directors, the Company has the ability to exercise significant influence over the operating and financial policies of this investee; therefore, the Company accounts for this investment as an equity method investment. The Company has no requirement to invest further in this private company and the ownership percentage may be diluted in the future. The Company will account for this investment as an equity method investment until the investment no longer meets the definition of an equity method investment.

The Company accounts for its share of the earnings or losses of the investee with a reporting lag of three months beginning the third quarter of 2018, as the financial statements of the investee are not completed on a basis that is sufficient for the Company to apply the equity method on a current basis. The Company has recorded $0.3 million of its share of losses of the investee as of December 31, 2018.

Property and Equipment, net

Property and equipment are stated at cost, net of accumulated depreciation and amortization. The cost of property and equipment is depreciated or amortized using the straight-line method over the respective useful lives of the assets, ranging from three to five years. Leasehold improvements are amortized using the straight-line method over the shorter of the estimated useful life of the asset or the lease term. Long-lived assets, including property and equipment are reviewed for impairment whenever events or circumstances indicate that the carrying amount of these assets may not be recoverable. The determinants used for this evaluation include management’s estimate of the asset’s ability to generate positive income from operations and positive cash flow in future periods, as well as the strategic significance of the assets to the Company’s business objectives. The Company did not recognize any impairment losses for the years ended December 31, 2018 or 2017.

Comprehensive Income/Loss

Comprehensive income/loss is defined as the change in shareholders’ equity during a period from transactions and other events and circumstances from non-owner sources. Comprehensive loss represents unrealized losses on the Company’s marketable securities. The income tax effect related to unrealized losses was immaterial for December 31, 2018 or 2017.

Revenue Recognition

The Company recognizes revenue when each of the following four criteria is met: (i) persuasive evidence of an arrangement exists; (ii) products are delivered or as services are rendered; (iii) the sales price is fixed or determinable; and (iv) collectability is reasonably assured.

Multiple-element arrangements may include (i) grants of licenses, or options to obtain licenses, to intellectual property, (ii) research and development services, (iii) participation on joint research or joint development committees, or (iv) manufacturing or supply services. Payments the Company may receive under these arrangements typically include one or more of the following: non-refundable upfront license fees, option exercise fees, funding of research or development efforts, amounts due upon the achievement of specified objectives, or royalties on future product sales.

Multiple-element arrangements require the separability of deliverables included in an arrangement into different units of accounting and the allocation of arrangement consideration to the units of accounting. The evaluation of multiple-element arrangements requires management to make judgments about (i) the identification of deliverables, (ii) whether such deliverables are separable from the other aspects of the contractual relationship, (iii) the estimated selling price of each deliverable, and (iv) the expected period of performance for each deliverable.

To determine the units of accounting under a multiple-element arrangement, management evaluates certain separation criteria, including whether the deliverables have stand-alone value, based on the relevant facts and circumstances for each arrangement. Management then estimates the selling price for each unit of accounting and allocates the arrangement consideration to each unit using the relative selling price method. The allocated consideration for each unit of accounting is recognized based on the method most appropriate for that unit of account and in accordance with the revenue recognition criteria detailed above.

If there are deliverables in an arrangement that are not separable from other aspects of the contractual relationship, they are treated as a combined unit of accounting, with the allocated revenue for the combined unit recognized in a manner consistent with the revenue recognition applicable to the final deliverable in the combined unit. Payments received prior to satisfying the relevant revenue recognition criteria are recorded as deferred revenue in the accompanying consolidated balance sheets and recognized as revenue when the related revenue recognition criteria are met.

Most of the collaboration agreements provide for non-refundable milestone payments. The Company recognizes revenue that is contingent upon the achievement of a substantive milestone in its entirety in the period in which the milestone is achieved. A milestone is considered substantive when the consideration payable to the Company for such milestone (i) is consistent with its performance necessary to achieve the milestone or the increase in value to the collaboration resulting from its performance, (ii) relates solely to its past performance and (iii) is reasonable relative to all of the other deliverables and payments within the arrangement. In making this assessment, management considers all facts and circumstances relevant to the arrangement, including factors such as the scientific, regulatory, commercial and other risks that must be overcome to achieve the milestone, the level of effort and investment required to achieve the milestone and whether any portion of the milestone consideration is related to future performance or deliverables.

Management periodically reviews the estimated performance periods under the collaboration agreements, which provide for non-refundable upfront payments and fees. Management adjusted the periods over which revenue was recognized when appropriate to reflect changes in assumptions relating to the estimated performance periods. In the first quarter of 2019, the Company will adopt new accounting guidance that will change future patterns of revenue recognition.

The Company records revenues related to the reimbursement of costs incurred under the collaboration agreements where it acts as a principal, controls the research or development activities and bears credit risk. Under its collaboration agreements, the Company is reimbursed for associated out-of-pocket costs and for a certain amounts of full-time equivalent, or FTE, costs based on an agreed-upon FTE rate. The gross amount of these pass-through reimbursed costs is reported as revenue in the accompanying consolidated statements of operations and comprehensive loss, while the actual expenses for which the Company is reimbursed are reflected as research and development costs.

Research and Development Costs, net

Research and development costs are expensed as incurred. These expenses result from the Company’s independent research and development efforts as well as efforts associated with collaboration arrangements. Research and development costs include salaries and personnel-related costs, consulting fees, fees paid for contract research and manufacturing services, the costs of laboratory supplies, equipment and facilities and other external costs are shown net of any grants.

Share-Based Compensation

The Company recognizes share-based compensation for equity awards granted to employees, officers, and directors as an expense on the statements of operations. Share-based compensation is recognized over the requisite service period of the individual awards using the straight-line attribution method, which generally equals the vesting period. Share options have a ten-year life and generally vest 25% on the first anniversary of the grant and in 1/48th equal installments on each monthly anniversary thereafter, such that options are fully vested on the four-year anniversary of the date of grant.

The fair value of share options is estimated using a Black-Scholes valuation model on the date of grant. This method requires certain assumptions be used as inputs, such as the fair value of the underlying common shares, expected term of the option before exercise, expected volatility of the Company’s Ordinary Shares, expected dividend yield, and a risk-free interest rate. The Company has limited historical share option activity and therefore estimates the expected term of share options granted using the simplified method, which represents the average of the contractual term of the share option and its weighted-average vesting period. The expected volatility of share options is based upon the historical volatility of a peer group of publicly traded companies. The Company has not declared or paid any dividends and do not currently expect to do so in the foreseeable future. The risk-free interest rates used are based on the implied yield currently available in United States Treasury securities at maturity with a term equivalent to the expected term of the share options. The effect of forfeited awards is recorded when the forfeiture occurs.

Share-based awards granted to non-employees are remeasured at each reporting date and compensation costs are recognized as services are rendered, generally on a straight-line basis. The Company believes that the fair value of these awards is more reliably measurable than the fair value of the services rendered. There were no share-based awards granted to non-employees during 2018 and 2017.

Ordinary Shares Valuation

Prior to the merger and due to the absence of an active market for the Company’s Ordinary Shares, the Company utilized third-party valuations which utilized methodologies in accordance with the framework of the American Institute of Certified Public Accountants Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation, to estimate the fair value of its Ordinary Shares.

Statement of cash flows

The following table provides a reconciliation of cash and cash equivalents and restricted cash reported within the consolidated balance sheets to the total of the same such amounts shown in the consolidated statement of cash flows:

	As of December 31,
(in thousands)	2018		2017
Cash and cash equivalents	$	36,709	$	24,965
Restricted cash		—		166
Non-current Restricted cash		107		107
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$	36,816	$	25,238

Income Tax Expense

Income taxes are accounted for using an asset and liability approach that requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the financial reporting basis and the tax basis of the Company’s assets and liabilities at the applicable tax rates, along with net operating loss and tax credit carryovers. The Company records a valuation allowance against its deferred tax assets to reduce the net carrying value to an amount that it believes is more likely than not to be realized. Management has considered estimated taxable income and ongoing prudent and feasible tax planning strategies in assessing the amount of the valuation allowance. Based upon the weight of available evidence, which includes the Company’s historical operating performance and limited potential to utilize tax credit carryforwards, the Company has determined that total deferred tax assets should be fully offset by a valuation allowance. When the Company establishes or reduces the valuation allowance against its deferred tax assets, its provision for income taxes will increase or decrease, respectively, in the period such determination is made.

The Company is required to file federal and state income tax returns in the United States and various other state jurisdictions. The Company also files income tax returns in the foreign countries in which it operates. The preparation of these income tax returns requires the Company to interpret the applicable tax laws and regulations in effect in such jurisdictions, which could affect the amount of tax paid by the Company.

Additionally, the Company follows an accounting standard addressing the accounting for uncertainty in income taxes that prescribes rules for recognition, measurement, and classification in the consolidated financial statements of tax positions taken or expected to be taken in a tax return.

The Tax Cuts and Jobs Act (the Act) was enacted on December 22, 2017 resulting in significant modifications to existing law. The Company follows the guidance of Staff Accounting Bulletin (SAB) 118, which provides additional clarification regarding the application of ASC 740 in situations where the Company does not have the necessary information available, prepared or analyzed in reasonable detail to complete the accounting for certain income tax effects of the Act for the reporting period in which the Act was enacted. SAB 118 provides for a measurement period beginning in the reporting period that includes the Act’s enactment and ending when the Company has obtained, prepared and analyzed the information needed in order to complete the accounting requirements, but in no circumstances should the measurement period extend beyond one year from the enactment date.

The Company has completed its analysis of the Act’s income tax effects. In total, the Company recorded $2.4 million related to the remeasurement of deferred tax assets which was fully offset by a corresponding decrease in the valuation allowance.

Net Loss per Share

Basic net loss per share is calculated by dividing the net loss by the weighted-average number of Ordinary Shares outstanding for the period, without consideration for ordinary share equivalents. Diluted net loss per share is calculated by dividing the net loss by the weighted-average number of Ordinary Shares and dilutive ordinary share equivalents outstanding for the period determined using the treasury-stock method. Dilutive Ordinary Shares for the year ended December 31, 2018 are comprised of share options. For the year ended December 31, 2017, dilutive Ordinary Shares are comprised of options, convertible preferred stock, convertible notes, and warrants.

No dividends were declared or paid during the reported periods.

Recently Issued Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (FASB) amended the existing Accounting Standards Update (ASU) for revenue recognition No. 2014-09, Revenue from Contracts with Customers (Topic 606), which outlines a comprehensive revenue recognition model and supersedes most current revenue recognition guidance. ASU 2014-09 outlines a five-step process for revenue recognition that focuses on transfer of control, as opposed to transfer of risk and rewards, and also requires enhanced disclosures regarding the nature, amount, timing and uncertainty of revenues and cash flows from contracts with customers. Major provisions include determining which goods and services are distinct and require separate accounting (performance obligations), how variable consideration (which may include change orders and claims) is recognized, whether revenue should be recognized at a point in time or over time and ensuring the time value of money is considered in the transaction price.

The FASB subsequently issued amendments to ASU No. 2014-09 that have the same effective date and transition date. These new standards will become effective for the Company on January 1, 2019. The Company will implement the new guidance using the modified retrospective approach. The Company has performed a review of these new standards as compared to its current accounting policies for collaborative relationships. The Company is evaluating the impact of the new standard on historical revenue recorded for its collaboration agreements. This ongoing evaluation is dependent upon the resolution of certain questions relating to the application of the new revenue recognition guidance for collaboration agreements which will ultimately determine the impact, if any, the adoption of this standard may have on the consolidated financial statements.

In February 2016, the FASB issued ASU 2016-02, Leases, which outlines a comprehensive lease accounting model and supersedes the current lease guidance. The new accounting standard requires lessees to recognize lease liabilities and corresponding right-of-use assets for all leases with lease terms of greater than twelve months. It also changes the definition of a lease and expands the disclosure requirements of lease arrangements. The new accounting standard must be adopted using the modified retrospective approach and is effective for entities for annual reporting periods beginning after December 15, 2018, with early adoption permitted.

In July 2018, the FASB issued ASU 2018-11, Leases (Topic 842): Targeted Improvements, provides entities an optional transition method to apply the new guidance as of the adoption date, rather than as of the earliest period presented. In transition, entities may also elect a package of practical expedients that must be applied in its entirety to all leases commencing before the effective date, unless the lease modified, to not reassess (a) the existence of a lease, (b) lease classification or (c) determination of initial direct costs, which effectively allows entities to carryforward accounting conclusions under previous U.S. GAAP. The Company will adopt the standard on January 1, 2019, using the optional transition method to apply the new guidance as of January 1, 2019, rather than as of the earliest period presented, and elect the package of practical expedients described above.

The Company expects Topic 842 will have a material effect on its consolidated balance sheet. However, the Company does not expect Topic 842 will have a material effect on its consolidated statements of operations and comprehensive loss or consolidated statements of cash flows. While the Company continues to assess all of the effects of adoption, the most significant effects relate to (1) the recognition of right-of-use (ROU) assets and lease liabilities within a range of approximately $5.5 million to $6.5 million using an assumed incremental borrowing rate of 8.4%, primarily resulting from leases of office and laboratory space; (2) the derecognition of deferred rent of approximately $0.5 million for certain lease incentives received; and (3) significant new disclosure requirements.

In June 2018, the FASB issued ASU No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, which amends the FASB Accounting Standards Codification in order to simplify the accounting for share-based payments granted to nonemployees for goods and services. Under the ASU, most of the guidance on such payments to nonemployees will be aligned with the requirements for share-based payments granted to employees. The guidance mandates the modified retrospective approach and is effective for annual and interim reporting periods beginning after December 31, 2018, with early adoption permitted. The Company plans to adopt this guidance in the first quarter of 2019 and does not expect adoption will have a material impact on the Company’s consolidated financial statements.

Recently Adopted Accounting Pronouncements

Effective January 1, 2017, the Company adopted ASU No. 2017-09 Compensation – Stock Compensation (Topic 718): Scope of Modification Accounting (ASU No. 2017-09). ASU No. 2017-09 provides guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting. The Company’s adoption of ASU No. 2017-09 had no impact on the Company’s statements of financial position or results of operations and comprehensive loss.

In November 2016, the FASB issued ASU 2016-18, Restricted Cash, which requires entities to show the changes in the total of cash, cash equivalents, restricted cash and restricted cash equivalents in the statement of cash flows. As a result, entities will no longer present transfers between cash and cash equivalents and restricted cash and restricted cash equivalents in the statement of cash flows. This update is effective for annual periods beginning after December 15, 2017, and interim periods within those fiscal years with early adoption permitted. The Company adopted this pronouncement retrospectively effective in the December 31, 2017 consolidated financial statements. There was no effect on previously reported balances as a result of adoption of the standard.

In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): “Clarifying the Definition of a Business” which clarifies the definition of a business and affects all companies and other reporting organizations that must determine whether they have acquired or sold a business. The amendments are intended to assist with the evaluation of whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The guidance is effective for the Company for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years and should be applied prospectively as of the beginning of the period of adoption. Early adoption is permitted under certain circumstances. The Company adopted ASU 2017-01 as of January 1, 2017 and the adoption did not have an impact on the Company’s accounting and disclosures.

Collaboration Revenue

12 Months Ended

Dec. 31, 2018

Collaboration Agreements [Abstract]

Collaboration Revenue

NOTE 3. Collaboration Revenue

The Company has entered into license agreements and collaborative research and development arrangements with pharmaceutical and biotechnology companies. Under these arrangements, the Company is entitled to receive license fees, upfront payments, milestone payments if and when certain research and development milestones or technology transfer milestones are achieved, royalties on approved product sales and reimbursement for research and development activities. The Company’s costs of performing these services are included within research and development expenses. The Company’s milestone payments are typically defined by achievement of certain preclinical, clinical, and commercial success criteria. Preclinical milestones may include in vivo proof of concept in disease animal model(s), lead candidate identification, and completion of IND-enabling toxicology studies. Clinical milestones may, for example, include successful enrollment of the first patient in or completion of Phase I, II, and III clinical trials, and commercial revenue, milestone or royalty-based, is often tiered based on net or aggregate sale amounts. The Company cannot guarantee the achievement of these milestones and economics due to risks associated with preclinical and clinical activities required for development of nucleic acid medicine-based therapeutics.

The following table summarizes the Company’s collaboration revenues for the periods indicated (in thousands). Approximately $5.0 million and $1.0 million of total collaboration revenue represents revenue derived from foreign countries for the years ended December 31, 2018 and 2017, respectively.

	Year Ended December 31,
(Dollars in thousands)	2018		2017
Collaboration Partner - Janssen	$	1,232	$	4,862
Collaboration Partner - Ultragenyx		6,794		5,639
Collaboration Partner - Takeda		1,137		1,403
Collaboration Partner - CureVac		4,427		—
Other		2,163		1,094
	$	15,753	$	12,998

The following paragraphs provide information on the nature and purpose of these collaboration arrangements.

Collaboration Partner – Janssen

Janssen 2015 Agreements

In 2015 the Company entered into two agreements with Janssen. The primary focus of the collaboration is to develop nucleic acid therapeutics for Hepatitis B (HBV). The Company analyzed the form and substance of both of the agreements and concluded they should be evaluated as a single arrangement for accounting purposes. Upon execution of the agreements, the Company received an upfront payment of $2.0 million which was amortized over the estimated research and development period.

Under the 2015 agreements, the Company recognized revenue of $4.9 million during the year ended December 31, 2017. The revenue recognized as of December 31, 2017 included labor and expense reimbursements of $4.4 million with the remaining revenue representing the amortized portion of the upfront fee and milestone payment. During the quarter ended September 30, 2017, the 2015 agreement was terminated, and the remaining unamortized upfront payment was recognized as of September 30, 2017.

Janssen 2017 Agreement

In late-2017, the Company and Janssen entered into a new agreement. The Company reviewed the timing and nature of the arrangement upon the signing of the new agreement and determined that it was not linked to the prior agreements and should be considered as a standalone agreement.

The 2017 collaboration agreement allocated discovery, development, funding obligations, and ownership of related intellectual property among the Company and Janssen. The Company received an upfront payment of $7.7 million and may receive preclinical, development and sales milestone payments of $56.5 million, as well as royalty payments on any future licensed product sales. Janssen will reimburse the Company for research costs at a future defined period upon the achievement of the first research milestone. Janssen may also pay option exercise fees within the $1.0 million to $5.0 million range per target. Janssen will pay royalties on annual net sales of licensed products in the low to mid-single digits range, subject to reduction on a country-by-country and licensed-product-by-licensed-product basis and subject to certain events, such as expiration of program patents. In addition, the collaboration includes an exclusivity period.

As the license component of the contract has no standalone value, the license and the research and development activities, exclusivity, and joint steering committee obligations under this agreement should be considered as a single unit of accounting in the arrangement. The upfront fee of $7.7 million is being deferred and recognized as revenue using the proportional performance method as the Company determined that the deliverables are fulfilled in a pattern other than straight-line due to the structure and nature of the collaborative arrangement. Total deferred revenue as of December 31, 2018 and December 31, 2017 for Janssen was $6.5 million and $7.6 million, respectively. The Company recognized revenue of $1.2 million and $0.1 million for the years ended December 31, 2018 and 2017, respectively. The revenue recognized included labor and expense reimbursements of a negligible amount with the remaining revenue representing the recognized portion of the upfront fee.

Collaboration Partner – Ultragenyx

In 2015 the Company entered into an agreement with Ultragenyx. During the initial phase of the collaboration, the Company will design and optimize therapeutics for certain rare disease targets. Ultragenyx has the option to add additional rare disease targets during the collaborative development period. Additionally, during the collaborative development period, the Company will participate with Ultragenyx in a joint steering committee. In addition, the collaboration includes an initial exclusivity period and an option to extend this period.

For each program, Ultragenyx will reimburse the Company for all internal and external development costs incurred and if Ultragenyx achieves certain, clinical, regulatory and sales milestones, then the Company is eligible to receive additional payments.

As part of the agreement, Ultragenyx paid an upfront fee and agreed to certain research and development funding obligations. The Company is also entitled to certain additional payments upon exercise of the Ultragenyx expansion option and/or exclusivity extension (if any), and for costs incurred by the Company in conducting the activities assigned under each collaboration development plan. In addition, on a development target-by-development target basis during the two-year period from the effective date of contract, Ultragenyx will pay the Company a one-time milestone payment after the first optimized lead designation for the first product with respect of such development target. For each development target for which Ultragenyx exercises its option, Ultragenyx will pay the Company a one-time option exercise fee within the $2.0 million to $5.0 million range per development target. In 2018, the Company signed an amendment with Ultragenyx, that may reduce milestone payments dependent on whether the Company does not incorporate a predefined chemistry methodology to increase mRNA half-life.

The agreement included potential milestone payments for selected targets from Ultragenyx to the Company. The current potential milestone payment for the remaining target as of December 31, 2018 is $69.5 million. Ultragenyx will pay royalties as a percentage of net sales on a product-by-product and country-by-country basis during the applicable royalty term up to and including double digits. As of December 31, 2018, the Company has not yet reached the clinical phase of the contract.

The Company concluded that the license, research and development activities, exclusivity, and joint steering committee obligations under this agreement should be considered a single unit of accounting in the arrangement, and the upfront fee of $10 million is being deferred and recognized as revenue over the same period as the estimated research activities. As of December 31, 2018, the amortization period is currently expected to end on March 31, 2019.

During 2017, the Company entered into an amendment with Ultragenyx to add one year to the exclusivity period for the reserved targets, in consideration for a one-time payment of $2.0 million. The extension of the exclusivity period did not change the length of the research and development period. Further, the amendment allows Ultragenyx the opportunity to review and comment on its filings and prosecution efforts of pending Company patents that relate to Ultragenyx chemistry. Since the Company’s deliverables under the agreement are considered a single unit of accounting, the payment consideration was added to the unamortized portion of the upfront signing fee and is being recognized systematically, on a straight-line basis, over the remainder of the period that the research and development services are expected to occur. During the fourth quarter of 2018, Ultragenyx extended the exclusivity on a specified number of reserved targets for an additional year with an annual reserve target list maintenance fee. This extension fee was deferred and will be recognized on a straight-line basis over the one-year exclusivity period. Total deferred revenue as of December 31, 2018 and December 31, 2017 for Ultragenyx was $2.7 million and $5.8 million, respectively.

The Company recognized revenue for Ultragenyx of $6.8 million and $5.6 million during the years ended December 31, 2018 and 2017, respectively. The revenue recognized included labor and expense reimbursements of $2.3 million and $3.7 million for the years ended December 31, 2018 and 2017, respectively, with the remaining revenue representing the amortized portion of the upfront fee on the arrangement.

Collaboration Partner – Takeda

In 2016 the Company entered into a contract with Millennium Pharmaceuticals, Inc., a wholly owned subsidiary of Takeda Pharmaceutical Company Limited (“Takeda”) to perform certain discovery and development of RNA medicines for treatment of nonalcoholic fatty liver disease (NASH). The agreement provided a non-exclusive license of the Company’s technology to Takeda for the 18 month research program term. As part of the agreement, Takeda paid an upfront fee of $0.1 million upon contract execution and agreed to provide the Company with funding for the discovery and development costs. The Company concluded that the research funding, exclusivity and license fees were to be accounted for as a single unit of accounting and the upfront license fees were deferred and recognized as revenue over the same period as the initial research program term. In 2017, the Company and Takeda amended the agreement to extend the research program scope and term through December 20, 2018.

The amended agreement provided for $3.7 million in regularly scheduled research funding payments through 2018. The scheduled fees paid are contractually refundable to Takeda if unearned by the Company. As noted above, the agreement ended during December of 2018 in accordance with the contractually agreed upon research term.

On March 8, 2019, the Company entered into a Research Collaboration Agreement with Takeda for the purpose of designing, optimizing and manufacturing LUNAR®-formulated mRNA Therapeutics. The Company recognized revenue from Takeda of $1.1 million and $1.4 million for the years ended December 31, 2018 and 2017, respectively. The revenue recognized includes expense reimbursements that are recognized as revenue when incurred as per the terms of the agreement and milestone payments.

Collaboration Partner - CureVac

In January 2018, the Company entered into a Development and Option Agreement with CureVac, (the “Development and Option Agreement”). Under the terms of the Development and Option Agreement, the parties have agreed to conduct joint preclinical development programs once CureVac makes a payment to pull down a target on the basis of which CureVac is granted options for taking a license on pre-agreed license terms to develop and commercialize certain products incorporating the Company’s patents and know-how related to delivery systems based on or incorporating lipid-mediated delivery systems (including the LUNAR^® platform) (the “Arcturus LMD Technology”), and CureVac patents and know-how related to mRNA technology. Subject to certain restrictions, the parties will have an undivided one-half interest in the patents and know-how developed jointly by the parties during the course of the Development and Option Agreement. Pursuant to the terms of the Development and Option Agreement, CureVac will have a number of target options to co-develop from a reserved target list to enter into licenses under the Arcturus LMD Technology with respect to the development, manufacture and commercialization of licensed products (which can include products identified for development by the Company unless the Company is permitted by the terms of the Development and Option Agreement to place such products on a restricted list). A separate notice and fee will be required for each license agreement. If the target to which the license agreement relates is chosen by the parties for co-development under the Co-Development Agreement (which is defined below and discussed in the following paragraph) the license agreement will terminate as such programs will be covered under the Co-Development Agreement discussed below, and therefore CureVac will be given a credit for any exercise fees, milestone payments already paid and all other payments made in relation to the license agreement towards future such payments incurred with respect to future licenses under the Arcturus LMD Technology. Pursuant to a May 2018 amendment to the Development and Option Agreement (which was amended and restated on September 28, 2018), the Company increased the number of targets available to CureVac under the Development and Option Agreement and agreed upon the license forms to be executed upon selection of the targets by CureVac.

Concurrently with the Development and Option Agreement, the Company entered into a Co-Development and Co-Commercialization Agreement (the “Co-Development Agreement”). Under the terms of such agreement, the parties will collaborate to develop and commercialize mRNA-based products for treating ornithine transcarbamylase (“OTC”) deficiency, incorporating CureVac’s mRNA technology, the Company’s mRNA technology and the Arcturus LMD Technology. The overall collaboration with CureVac was managed by a joint steering committee. The parties also have the option to co-develop two mRNA programs for CureVac and one mRNA program for the Company that are not included on the target escrow list. All costs incurred from the Co-Development Agreement will be shared equally by the Company and CureVac, and any costs incurred by the Company in excess of CureVac will be divided evenly and recognized as revenue. The Company recognized $3.3 million of collaboration revenue related to the Co-Development Agreement as of December 31, 2018.

The Company concluded that the contracts should be accounted for on a combined basis due to their being negotiated and signed concurrently as well as the interoperability between the two agreements, and that the research and development activities, exclusivity, license fees, governance and reserve target rights were to be accounted for as a single unit of accounting and the upfront license fees of $5.0 million were deferred and recognized as revenue over the eight-year research program period. Further, the Company concluded that the options granted to CureVac are substantive and are to be evaluated as a separate arrangement and not a deliverable of the original arrangement since the option is at the sole election of the customer and due to the fact that the exercise price for the option is reasonable in comparison to other payments in the arrangement. Total deferred revenue as of December 31, 2018 was $4.4 million under the Development and Option Agreement. The Company recognized revenue from CureVac under both agreements of $4.4 million for the year ended December 31, 2018. The revenue recognized included labor and expense reimbursements of $3.8 million for the year ended December 31, 2018, with the remaining revenue representing the amortized portion of the upfront fee under the Development and Option Agreement.

On February 11, 2019, the Company announced the termination of the obligations of CureVac under the Co-Development Agreement, effective 180 days from February 5, 2019 and the re-assumption by the Company of the worldwide rights thereto. Arcturus will reassume 100% global rights for its flagship asset, clinical development candidate ARCT-810, a messenger RNA (mRNA) drug to treat OTC deficiency. ARCT-810 was previously subject to equal cost sharing between Arcturus and CureVac under the Co-Development Agreement. CureVac elected not to continue its obligations for the preclinical development of ARCT-810 under and pursuant to the terms of the agreement.

Pursuant to the terms of the Co-Development Agreement, CureVac is obligated to continue to fund its share of the preclinical expenses for the OTC program until August of 2019.

Other Collaboration Agreements

The Company entered into several other smaller agreements and recorded revenue and deferred revenue consistently with the revenue recognition practices described in the significant accounting policies footnote. The total revenue of $2.2 million from other smaller agreements was primarily related to a Research and Exclusive License Agreement with Synthetic Genomics, Inc. (“SGI”) which the Company entered into during the fourth quarter of 2017. Under the agreement, the Company granted SGI an exclusive license for the Arcturus LMD Technology to research, develop and sell products for diseases excluding all respiratory disease viruses other than influenza. Revenue related to this agreement is made up of labor reimbursements and sublicense revenue. The sublicense revenue is calculated as a percentage of all cash payments received by SGI from any sublicense for a LUNAR product, in the mid 10% to 20% range, less payments made to third parties to obtain the right to practice intellectual property used to develop or necessary to make, use, or sell all or part of licensed LUNAR product. Under certain circumstances, the Company will be owed a percentage ranging from 5% to 10% of amounts received by SGI should they enter into agreements. Additionally, in order to maintain exclusive rights, SGI must achieve certain specified sublicense milestones or pay the Company annual exclusivity maintenance fees. As part of the agreement, SGI paid an upfront fee of $0.2 million upon contract execution which is creditable against any payments to Arcturus. Therefore, the upfront fee was fully deferred upon the receipt of funds. The Company recognized $1.4 million of labor reimbursement and sublicense revenue as of December 31, 2018.

The remaining revenue from smaller collaboration agreements primarily relates to the agreement with Providence Therapeutics, Inc. (“Providence”), a related party. Under this agreement, the Company recognized revenue of $0.6 million from amortization of an upfront payment, labor reimbursements, and out-of-pocket cost reimbursements. See Note 14 Related Party Transactions for further details of the Providence agreement.

Short-term Investments

12 Months Ended

Dec. 31, 2018

Investments Debt And Equity Securities [Abstract]

Short-term Investments

NOTE 4. Short-term Investments

As of December 31, 2017, the Company’s short-term investments consisted of short-term bank deposits and marketable securities totaling $23.6 million. The balance of investments as of December 31, 2017 included bank deposits of $15.0 million with maturities of more than three months but less than one year as well as short-term deposits stated at cost which approximated market value. As of December 31, 2017, the Company’s short-term bank deposits bore interest at a weighted average annual interest rate of 1.6%. During 2018, all short-term bank deposits were liquidated.

There were no short-term investments as of December 31, 2018. The following is a summary of short-term investments at December 31, 2017:

	December 31, 2017
(Dollars in thousands)	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair value
Certificates of deposit	$	1,462	$	—	$	—		$	1,462
Corporate debt securities		7,149		—		(3	)		7,146
Total	$	8,611	$	—	$	(3	)	$	8,608

All short-term investments are held as available-for-sale and mature within twelve months of December 31, 2017.

Fair Value Measurements

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

NOTE 5. Fair Value Measurements

The Company establishes the fair value of its assets and liabilities using the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. The Company established a fair value hierarchy based on the inputs used to measure fair value.

The three levels of the fair value hierarchy are as follows:

Level 1: Quoted prices in active markets for identical assets or liabilities.

Level 2: Inputs, other than the quoted prices in active markets, that are observable either directly or indirectly.

Level 3: Unobservable inputs in which little or no market data exists, and are therefore determined using estimates and assumptions developed by the Company, which reflect those that a market participant would use.

The carrying value of cash, restricted cash, short-term bank deposits, accounts receivable, accounts payable, and accrued liabilities approximate their respective fair values due to their relative short maturities.

As of December 31, 2018, all assets measured at fair value on a recurring basis consisted of cash equivalents which were classified within Level 1 of the fair value hierarchy. The following table presents the fair value hierarchy for assets measured at fair value on a recurring basis as of December 31, 2017 (in thousands):

	December 31, 2017
	Fair value measurements using input type
	Level 1		Level 2		Level 3		Total
Cash equivalents	$	2,024	$	—	$	—	$	2,024
Certificates of deposit		—		1,462		—		1,462
Corporate debt securities		—		7,146		—		7,146
Total financial assets	$	2,024	$	8,608	$	—	$	10,632

The fair value of certain financial instruments was measured and classified within Level 1 of the fair value hierarchy based on quoted prices. Certain financial instruments classified within Level 2 of the fair value hierarchy include the types of instruments that trade in markets that are not considered to be active, but are valued based on quoted market prices, broker or dealer quotations, or alternative pricing sources with reasonable levels of price transparency.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Reverse Merger with Alcobra Ltd.

12 Months Ended

Dec. 31, 2018

Business Combinations [Abstract]

Reverse Merger with Alcobra Ltd.

NOTE 6. Reverse Merger with Alcobra Ltd.

As described in Note 1 “Organization,” the reverse merger completed between Arcturus and Alcobra Ltd. was accounted for as an issuance of shares by the Company for the net assets of Alcobra Ltd., accompanied by a recapitalization. Arcturus was considered the acquirer for accounting and financial reporting purposes and acquired the assets and assumed the liabilities of Alcobra Ltd. Arcturus gained control of the combined company after the merger. The annual consolidated financial statements of the Company reflect the operations of the acquirer for accounting purposes together with a deemed issuance of shares, equivalent to the shares held by the former stockholders of the legal acquirer and a recapitalization of the equity of the accounting acquirer. The annual consolidated financial statements include the accounts of the Company since the effective date of the reverse capitalization and the accounts of Arcturus Therapeutics, Inc. since inception.

The following summarizes the estimated fair value of the assets and liabilities acquired at November 15, 2017, the date of the merger:

(in thousands)
Cash and cash equivalents	$	2,032
Restricted cash		179
Short-term investments		34,188
Prepaid and other assets		434
Property, plant and equipment – held for sale		29
Intangible asset-held for sale		590
Total assets acquired		37,452
Accounts payable and accrued expenses		(1,906	)
Net assets acquired	$	35,546

The estimated fair value of total considerations paid was $40,841,000 based on the shares and options of Alcobra Ltd. outstanding on the merger date as adjusted per the merger agreement of 3,997,000 multiplied by the closing price of $10.22 on the date of the merger. The excess of the fair value of the consideration paid over the fair value of the net assets acquired as detailed above was $5,295,000, which was recorded as a charge to additional paid-in capital in the equity section of the consolidated balance sheet. The Company also incurred direct merger-related costs totaling $1,734,000, which offset proceeds received from the transaction and were recorded as a reduction to additional paid-in capital on the consolidated balance sheet.

Assets acquired in the merger included an intangible asset consisting of in-process research and development for proprietary drug technology called ADAIR. At the closing date of the reverse merger, the Company entered into an agreement with Amiservice to which the Company agreed to transfer certain intellectual property related to ADAIR in exchange for a minority equity stake in a company to be formed by Amiservice for the purpose of acquiring the ADAIR assets. The Company determined that the asset met the classification criteria as held for sale in accordance with related accounting guidance when acquired and remained held for sale at December 31, 2017. To determine the fair value of the ADAIR asset, the Company utilized an independent valuation consultant who valued the asset using a market approach valuation method. In conjunction with this valuation, management judgment was required to forecast the occurrence of future events that would trigger the closing of the ADAIR sale agreement.

At the end of the second quarter of 2018, the Company completed the sale of its intangible assets related to the ADAIR technology (Note 2).

Balance Sheet Details

12 Months Ended

Dec. 31, 2018

Balance Sheet Related Disclosures [Abstract]

Balance Sheet Details

NOTE 7. Balance sheet details

Accrued liabilities consisted of the following as of December 31, 2018 and December 31, 2017.

	December 31,
(in thousands)	2018		2017
Accrued compensation	$	974	$	1,812
Refundable fees received		2,259		—
Other accrued liabilities		674		981
Total	$	3,907	$	2,793

Property and Equipment, Net

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Property and Equipment, Net

NOTE 8. Property and Equipment, Net

Property and equipment, net consisted of the following:

	December 31,
(in thousands)	2018			2017
Research equipment	$	2,711		$	1,620
Computers and software		200			97
Office equipment and furniture		527			255
Leasehold improvements		34			44
Total		3,472			2,016
Less accumulated depreciation and amortization		(1,497	)		(967	)
Property and equipment, net	$	1,975		$	1,049

Depreciation and amortization expense was $582,000 and $410,000 for the years ended December 31, 2018 and December 31, 2017, respectively.

Debt

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Debt

NOTE 9. Debt

Long-term debt with Western Alliance Bank

On October 12, 2018, the Company entered into a Loan and Security Agreement with Western Alliance Bank whereby the Company received gross proceeds of $10.0 million under a long-term debt agreement (the “Loan”). The Loan has a maturity date of October 1, 2022 and carries interest at the U.S. prime rate plus 1.25%. The loan has an interest-only period of 19 months, which could be extended by an additional 6 months if certain conditions are met, followed by an amortization period of 30 months, or 24 months if the interest-only period is extended. As of December 31, 2018, the Company estimated that the interest-only period will be 19 months, followed by an amortization period of 30 month.

The Company paid a loan origination fee of $128,000 which was recorded as a debt discount and is being accreted over the term of the Loan. In addition, the Company is required to pay a fee of $350,000 upon certain change of control events.

Upon maturity or prepayment, the Company will be required to pay a 3% fee, or a 2% fee if the U.S. Food and Drug Administration accepts certain Investigational New Drug (“IND”) applications prior to maturity. Because acceptance of an IND is outside of the Company’s control, management estimated that the Company will be liable for a fee of 3% of the principal balance, or $300,000 upon repayment or maturity, and such fee is accreted to the debt balance using the effective interest method over the term of the Loan.

The Loan is collateralized by all of the assets of the Company, excluding intellectual property, which is subject to a negative pledge. The Loan contains customary conditions of borrowing, events of default and covenants, including covenants that restrict the Company’s ability to dispose of assets, merge with or acquire other entities, incur indebtedness and make distributions to holders of the Company’s capital stock. In addition, the Company is required to maintain at least 50% of its deposit and investment accounts, or $20 million, whichever is lower, with the Western Alliance Bank.

The Loan also includes covenants which include the Company’s (1) nomination of a clinical candidate by December 31, 2018, which the Company is in compliance with, and (2) submission of a clinical candidate for Investigational New Drug application (“IND”), made to the U.S. Food and Drug Administration by December 31, 2019 and have it approved by January 31, 2020, provided that, if the Company has received net cash proceeds from sale, on or after October 12, 2018, of the Company’s equity securities in an amount of not less than $15,000,000, then the IND submission date shall be extended to May 31, 2020 and the approval date shall be extended to June 30, 2020.

Should an event of default occur, including the occurrence of a material adverse effect, the Company could be liable for immediate repayment of all obligations under the Loan. As of December 31, 2018, the Company is in compliance with all covenants and conditions of the Loan.

As of December 31, 2018, the outstanding long-term debt balance was $9.9 million, inclusive of $28,000 of accretion of the final payment and net of the unamortized debt discount of $117,000.

Principal payments, including the final payment due at repayment, on the long-term debt are as follows as of December 31, 2018:

Year Ending December 31,
2019	$	—
2020		2,666,667
2021		4,000,000
2021		3,633,333
Total		10,300,000

Convertible Promissory Notes

On November 15, 2017 and in connection with the merger, holders of all of the Company’s convertible promissory notes that were issued in the second quarter of 2017 converted $5,795,000 of principal value and $162,000 of accrued interest into 616,824 Ordinary Shares at an average conversion rate of $10.19 per share. Additionally, the Company recognized additional expense of $348,000 as a result of the beneficial conversion feature received by the noteholders upon settlement per terms of the amended note agreements, which was charged to finance expense included in the consolidated statements of operations and comprehensive loss.

The Company recognized interest expense related to its long-term debt of $186,000 and $150,000 during the years ended December 31, 2018 and 2017, respectively.

Shareholders’ Equity

12 Months Ended

Dec. 31, 2018

Stockholders Equity Note [Abstract]

Shareholders’ Equity

NOTE 10. Shareholders’ Equity

Ordinary Shares

Merger and reverse stock split

The Company completed the merger with Alcobra Ltd. on November 15, 2017 as described in Note 6 to the consolidated financial statements. In connection with the merger, all outstanding shares of Arcturus Therapeutics, Inc. were exchanged for the Company’s Ordinary Shares at a rate of 0.293 Ordinary Shares of the Company’s stock for each share of Arcturus Therapeutics, Inc. common stock.

Also on November 15, 2017 and prior to and in connection with the merger, Alcobra Ltd. effected a 1-for-7 reverse stock split of Ordinary Shares and changed Ordinary Shares authorized to 30,000,000 shares. All historical information presented herein has been retroactively restated to reflect the effect of the merger exchange ratio, reverse stock split and change to the authorized number of Ordinary Shares in accordance with Accounting Standards Codification Topic 260, “Earnings Per Share”.

Restricted Ordinary Shares

In March 2013, the founders of the Company purchased 2,783,686 Ordinary Shares of stock for $0.0068 per share. Of the shares purchased, 1,538,353 were subject to a repurchase option whereby the Company has an option for two months after date of termination of service to repurchase any or all of the unvested shares at the original purchase price per share. The repurchase option shall be deemed to be automatically exercised by the Company as of the end of the two-month period unless the Company notifies the purchaser that it does not intend to exercise its option. The shares will be vested (1) 25% after obtaining suitable siRNA license; (2) 25% after in vivo proof-of-concept achieved; (3) 25% after a regulatory agency new drug application (such as an Investigational New Drug application) is filed and accepted by the applicable regulatory agency; and (4) 25% after human biological proof-of-concept is achieved. The Company met the first two milestones during 2013 and 2014 leaving an unvested balance of 769,176 Ordinary Shares. In 2017, the Ordinary Shares purchase agreements were amended to clarify vesting conditions and also to accelerate the vesting of 146,510 Ordinary Shares resulting in a modification expense of $1,495,000 as of December 31, 2017. As of December 31, 2018 and 2017, there were 622,667 Ordinary Shares unvested and subject to the repurchase option.

Warrants

Warrants were issued in connection with the issuance of the Company’s preferred stock. In 2017 and in conjunction with the merger, all 192,647 of the outstanding warrants were exercised for 188,980 Ordinary Shares (after subtraction of shares for net exercise, when selected). The Company received proceeds of $160,000 in conjunction with the warrant exercises in 2017.

Net Loss per Share

Dilutive securities at December 31, 2018 that were not included in the calculation of diluted net loss per share for the year ended December 31, 2018 as they were anti-dilutive totaled 94,000. Dilutive securities that were not included in the calculation of diluted net loss per share because they were anti-dilutive totaled 3,057,000 potential shares at December 31, 2017.

For the years ended December 31, 2018 and 2017, the calculation of the weighted-average number of shares outstanding excludes both unvested restricted Ordinary Shares of 622,667 and shares held in treasury of 43,000. In addition, for only the year ended December 31, 2017, the calculation of weighted-average number of shares outstanding excludes 79,000 shares which were issued upon the early exercise of share options and subject to future vesting.

Share-Based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Share-Based Compensation

NOTE 11. Share-Based Compensation

Arcturus Therapeutics, Inc. had one stock compensation plan prior to the merger, the 2013 Equity Incentive Plan (the “2013 Plan”) which provides for the granting of options, warrants, restricted stock awards, restricted stock units, and other equity-based compensation to the Company’s directors, employees and consultants. In connection with the merger and as required in the 2013 Plan, all outstanding options in the 2013 Plan converted into options to purchase Alcobra Ltd.’s Ordinary Shares, as renamed Arcturus Therapeutics Ltd., and the applicable share amounts and exercise prices were adjusted to reflect the exchange ratio. The 2013 Plan has been extinguished and no additional grants shall be made from the 2013 Plan. Options granted under the 2013 Plan generally expire ten years from the date of grant. There were 38,751 shares available for future issuance under the 2013 Plan at December 31, 2018.

Prior to the merger, Alcobra Ltd. granted options to officers, directors, advisors, management and other key employees through the 2010 Incentive Option Plan (the “2010 Plan”). Substantially all options that were outstanding under the 2010 Plan became fully vested upon the closing of the merger. The value of these options was included as a component of the purchase price recorded in conjunction with the merger. The number of shares subject to and the exercise prices applicable to these outstanding options were adjusted in connection with the 1- for- 7 reverse share split. Options granted under the 2010 Plan generally expire ten years from the date of grant. Upon merger, the 2013 Plan was assumed by the 2010 Plan. The Company generally issues new shares upon option exercise. There were 94,001 shares available for future issuance under the 2010 Plan as of December 31, 2018; however, the Company does not intend to issue additional shares under the 2010 Plan.

In August 2018, the Company adopted the 2018 Omnibus Equity Incentive Plan (“2018 Plan”). Under the 2018 Plan, the Company is authorized to issue up to a maximum of 1,100,000 Ordinary Shares pursuant to the exercise of incentive share options or other awards provided for therein. In August 2018, the Company issued a certain number of options to purchase Ordinary Shares to a group of employees as well as options to purchase a total of 220,000 Ordinary Shares to its executives. The Company also issued options to purchase a total of 130,000 Ordinary Shares to the non-executive members of the Company’s board of directors. As of December 31, 2018, there were 470,000 shares available for future issuance under the 2018 Plan.

Share Options

The following table presents the weighted-average assumptions used in the Black-Scholes valuation model by the Company in calculating the fair value of share options granted:

	For the Year Ended December 31,
	2018			2017
Expected life (in years)		6.07			7.3
Expected volatility		73.3	%		76.4	%
Expected dividend yield		—	%		—	%
Risk-free interest rate		2.77	%		1.87	%
Grant date weighted average fair value	$	5.38		$	7.94

The following table summarizes the Company’s share option activity for the year ended December 31, 2018:

	Number of Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (in thousands)
Outstanding – December 31, 2017		344,055		$	8.70
Granted		1,046,931		$	8.13
Exercised		(62,224	)	$	5.34			$	218
Forfeited/cancelled		(139,329	)	$	16.92
Outstanding – December 31, 2018		1,189,433		$	7.41		9.11	$	301
Exercisable – December 31, 2018		280,782		$	5.77		7.53	$	255
Exercisable and expected to vest – December 31, 2018		1,189,433		$	7.41		9.11	$	301

At December 31, 2018, the total unrecognized compensation cost of $4.4 million will be recognized over the weighted-average remaining service period of approximately 3.1 years. The fair value of the options vested during the years ended December 31, 2018 and 2017 was $972,000 and $669,000, respectively.

Certain options granted were exercised prior to vesting, and are subject to repurchase by the Company at the lower of the original issue price or fair value and will vest according to the respective option agreement. As of December 31, 2017, 35,595 options were exercised but still subject to future vesting. No shares remained subject to repurchase as of December 31, 2018.

Share-based compensation expenses included in the Company’s statements of operations and comprehensive loss for the years ended December 31, 2018 and 2017 were:

	For the Year Ended December 31,
(in thousands)	2018		2017
Research and development	$	566	$	38
General and administrative		693		2,132
Total	$	1,259	$	2,170

Share-based compensation expense for the year ended December 31, 2017 includes $1,495,000 of expense related to a modification of a restricted Ordinary Shares agreement as discussed in Note 10.

During 2017, the Company granted options for 58,600 shares to two board members at an exercise price below fair value at the grant date. The awards were subject to performance conditions based on closing the reverse merger with Alcobra Ltd. and execution of a facility lease. All of the options vested during 2017, and related expense of $568,000 is included in general and administrative expense for the year ended December 31, 2017 related to the awards.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

NOTE 12. Income Taxes

A reconciliation of loss before income taxes for domestic and foreign locations for the years ended December 31, 2018 and 2017 is as follows:

	For the Year Ended December 31,
(In thousands)	2018			2017
United States	$	(21,604	)	$	(10,820	)
Foreign		(181	)		(81	)
Total loss before income taxes	$	(21,785	)	$	(10,901	)

The Company accounts for income taxes in accordance with ASC 740-10, Accounting for Uncertainty in Income Taxes. The impact of an uncertain income tax position on the income tax return must be recognized at the largest amount that is more likely than not to be sustained upon audit by the relevant taxing authority. An uncertain tax position will not be recognized if it has less than 50% likelihood of being sustained.

A reconciliation of income tax expense for the years ended December 31, 2018 and 2017 is as follows (in millions):

	December 31,
	2018		2017
Beginning balance of unrecognized tax benefits	$	0.4	$	0.4
Settlement of prior period tax positions		—		—
Increase for prior period tax positions		—		—
Increase for current period tax positions		—		—
Ending balance of unrecognized tax benefits	$	0.4	$	0.4

Included in the balance of unrecognized tax benefits at both December 31, 2018 and 2017 is $0.4 million that could impact the Company’s effective tax rate, if recognized, subject to a valuation allowance. None of the unrecognized tax benefits currently impact the Company’s effective tax rate due to the full valuation allowance the Company has recorded against its deferred tax assets.

The Company is subject to taxation and files income tax returns in the United States, California and Israel. Currently, no historical years are under examination. The Company’s tax years from 2013 to date are subject to examination by the Israeli, U.S. and state taxing authorities due to the carryforward of unutilized net operating losses and research and development credits. The Company’s policy is to recognize interest expense and penalties related to income tax matters as income tax expense. As of December 31, 2018, there are unrecognized tax benefits of $0.2 million and $0.2 million for the United States and California, respectively. There was no tax related interest or penalties recognized for the years ended December 31, 2018 and 2017.

The Company does not anticipate any material changes to its unrecognized tax benefits within the next twelve months.

The significant components of deferred income taxes at December 31, 2018 and 2017:

	December 31,
(in thousands)	2018			2017
Deferred tax assets:
Net operating loss	$	8,399		$	25,101
Tax credits		35			35
Accrued liabilities		261			227
Deferred revenue		1,713			1,162
Depreciation and amortization		46			—
Share-based compensation		85			90
Total gross deferred tax assets		10,539			26,615
Deferred tax liabilities:
Depreciation and amortization		—			(96	)
Valuation allowance		(10,539	)		(26,519	)
Net deferred tax asset	$	—		$	—

The Company has established a valuation allowance against net deferred tax assets due to the uncertainty that such assets will be realized. The Company periodically evaluates the recoverability of the deferred tax assets. At such time as it is determined that it is more likely than not that deferred tax assets will be realizable, the valuation allowance will be reduced.

At December 31, 2018, the Company had federal and state net operating losses, or NOL, carryforwards of approximately $33.2 million and $30.2 million, respectively. The federal NOL carryforwards begin to expire in 2034, and the state NOL carryforwards begin to expire in 2034. The Company has foreign NOL carryforwards of approximately $89.0 million that do not expire and can be carried forward indefinitely. Due to the Company’s recent plan of the Redomiciliation, it is more likely than not that the foreign NOL will not be realized. As a result, the Company has removed the foreign NOL carryforwards from its deferred tax asset schedule and recorded a corresponding decrease to its valuation allowance beginning January 1, 2018.

Excluded from the deferred tax assets for the net operating losses are pre-acquisition Alcobra Inc. and Alcobra Ltd. federal and foreign losses of $0.3 million and $20.4 million, respectively. The Company does not believe these losses will be available to use in the future due to limitations under IRC Section 382, lack of operations in Israel where the NOLs were generated and contemplated restructuring.

At December 31, 2018, the Company had federal and state research and development credit carryforwards of approximately $0.2 million and $0.2 million, respectively. The federal credit carryforwards begin to expire in 2033, and the state credits carry forward indefinitely.

The Company has also incurred research and development expenses of $17.0 million and $15.9 million for the years ended December 31, 2018 and 2017, respectively. The Company believes that a portion of these expenditures will yield additional federal and California tax credits; however, the potential credits under the tax laws have not yet been calculated.

Pursuant to Internal Revenue Code of 1986, as amended (the Code) Sections 382 and 383, annual use of the Company’s federal and California net operating loss and research and development credit carryforwards may be limited in the event a cumulative change in ownership of more than 50% occurs within a three-year period. The Company has not completed a Code Section 382 analysis regarding the limitation of net operating loss carryforwards and other tax attributes. There is a risk that changes in ownership have occurred since Company’s formation. If a change in ownership were to have occurred, the NOL carryforwards and other tax attributes could be limited or restricted. If limited, the related asset would be removed from the deferred tax asset schedule with a corresponding reduction in the valuation allowance. Due to the existence of the valuation allowance, limitations created by future ownership changes, if any, related to the Company’s operations in the U.S. will not impact the Company’s effective tax rate.

A reconciliation of the federal statutory income tax rate to the Company’s effective income tax rate is as follows:

	For the Year Ended December 31,
	2018			2017
Federal statutory income tax rate		21.0	%		34.0	%
State income taxes, net of federal benefit		5.3	%		4.4	%
Foreign rate differential		(1.3	%)		0.2	%
Share-based compensation		(0.2	%)		—	%
Tax Cuts and JOBS Act		—	%		(22.0	%)
Change in tax rate		—	%		(8.3	%)
Change in valuation allowance		(20.7	%)		1.8	%
Other		(3.0	%)		(1.8	%)
Permanent differences		(1.1	%)		(8.3	%)
Provision for income taxes		—	%		—	%

The Tax Cuts and Jobs Act (the Act) was enacted in the U.S. on December 22, 2017. The Act reduced the corporate tax rate to 21% from 35% rate, required companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred and created new taxes on certain foreign-sourced earnings. In 2017, the Company recorded provisional amounts for certain enactment-date effects of the Act by applying the guidance in SAB 118 because it had not yet completed its enactment-date accounting for these effects.

SAB 118 measurement period

The Company applied the guidance in SAB 118 when accounting for the enactment-date effects of the Act in 2017. At December 31, 2017, the Company had not completed its accounting for all of the enactment-date income tax effects of the Act under ASC 740, Income Taxes. At December 31, 2018, the Company completed its accounting for all of the enactment-date income tax effects of the Act. As further discussed below, during 2018, the Company did not recognize adjustments to the provisional amounts recorded at December 31, 2017.

As of December 31, 2017, the Company remeasured certain deferred tax assets and liabilities based on the rates at which they were expected to reverse in the future (which was generally 21%), by recording a provisional amount of $2.4 million, which was fully offset by a valuation allowance of the same amount. Upon further analysis of certain aspects of the Act and refinement of its calculations during the 12 months ended December 31, 2018, the Company found no other adjustments were necessary.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

NOTE 13. Commitments and Contingencies

Cystic Fibrosis Foundation Therapeutics Funding agreement

The Company has received royalty bearing grants sponsored by Cystic Fibrosis Foundation Therapeutics, Inc. (“CFFT”). Should the awards result in a successful product, the Company will pay CFFT a specified payment amount in installments following commercialization based on a formula that is six times the total award amount, plus a payment equal to the awarded payments, within sixty days after aggregate net sales of the product exceed certain thresholds. Further, in the event of a license, sale or other transfer of the product or the Company’s development program technology (including a change of control transaction), the Company will pay CFFT a percentage of such transfer payments actually received by the Company or the Company’s shareholders (subject to a royalty cap). As of December 31, 2018, the Company has received $0.5 million in grants and has not had a successful product utilizing CFFT grants.

Operating Leases

The Company leases office and lab space for its corporate headquarters in San Diego, California under a noncancellable operating lease. The initial lease term ended February 2018 and had monthly rental payments with escalations during the term of the lease.

In October 2017, the Company entered into a new lease for office space adjacent to its previously occupied headquarters. The commencement of the lease began in March 2018 and the lease extends for approximately 84 months from the commencement date. Monthly rental payments are due under the lease and there are escalating rent payments during the term of the lease. The Company is also responsible for its proportional share of operating expenses of the building and common areas. In conjunction with the new lease, the Company will receive free rent for four months and received a tenant improvement allowance of $74,000. The lease may be extended for one five year period at then current market rate with annual escalations. The Company entered into an irrevocable standby letter of credit with the landlord for security of $96,000 upon executing the lease which is included (along with additional funds required to secure the letter of credit) in the balance of non-current restricted cash as of December 31, 2018.

For operating leases, minimum lease payments, including minimum scheduled rent increases, are recognized as rent expense on a straight-line basis over the lease term. Leasehold improvement incentives paid to the Company by the landlord are recorded as a deferred rent and amortized as a reduction of rent expense over the lease term. Rent expense totaled $1.1 million and $334,000 for the years ended December 31, 2018 and 2017, respectively.

Future minimum payments under leases and lease commitments with initial terms greater than one year were as follows at December 31, 2018 (in thousands):

2019	$	1,268
2020		1,277
2021		1,315
2022		1,350
2023		1,390
Thereafter		1,745
Total	$	8,345

Related Party Transactions

12 Months Ended

Dec. 31, 2018

Related Party Transactions

Note 14. Related Party Transactions

During 2016, the Company entered into a Research Collaboration and License Agreement with a related party, Providence, whose CEO and President is also a shareholder of the Company, to identify and optimize microRNA modulators and/or mimetics for the treatment of neoplastic diseases. In April 2017, the Providence Agreement was amended to include mRNA for the treatment of neoplastic disease. In July 2018, the Providence Agreement was amended and restated to cover brain neoplasms, breast neoplasms and ovarian neoplasms. Each party is responsible for their own research costs under the agreement, and Providence is responsible for all of the development costs through the completion of Phase 2 clinical trials. The Company is entitled to share in future product revenue of each product provided the Company shares in the product’s post Phase 2 costs. Separately, Providence has agreed to pay for FTEs at a specified rate. For the years ended December 31, 2018 and 2017, the Company has recognized $0.6 million and $1.0 million, respectively, in revenue related to the amortization of the upfront payment and revenue related to the payment for FTEs and expense reimbursements. There were no outstanding accounts receivable balances related to this agreement as of December 31, 2018 and December 31, 2017. During the third quarter of 2017, the Company’s ordinary share agreement for the President and CEO of Providence was modified to remove the vesting conditions of the original grant and the Company recognized $1.5 million in related stock compensation expense.

During May 2018, the Company agreed to reimburse professional fees related to the proxy incurred by Joseph Payne, President & CEO, Board of Director, and Padmanabh Chivukula, COO & CSO, totaling $1.4 million.

Litigation

12 Months Ended

Dec. 31, 2018

Loss Contingency Information About Litigation Matters [Abstract]

Litigation

Note 15. Litigation

Previously Disclosed Litigation in Israel and California and California Arbitration

The Company, executive officers Joseph Payne (CEO) and Padmanabh Chivukula (COO and CSO) and certain former and current members of the Company’s board of directors were party to now terminated lawsuits filed in Israel and California and an arbitration in California. These lawsuits and the arbitration emanated from disputes among certain of these parties in connection with actions taken by former board members to terminate the employment of Mr. Payne and Dr. Chivukula and lawsuits filed by Mr. Payne in response to these terminations.

Mr. Payne and Dr. Chivukula have been reappointed to their roles as CEO (Payne) and COO and CSO (Chivukula) with the Company and the Company entered into an Agreement and Release with its current officers and certain former directors and officers to terminate all of the then ongoing litigation in Israel and the Unites States and the arbitration that arose in connection with the terminations of Mr. Payne and Dr. Chivukula. Accordingly, all of the previously described lawsuits and the arbitration have been dismissed with prejudice.

Subsequent Events

12 Months Ended

Dec. 31, 2018

Subsequent Events [Abstract]

Subsequent Events

Note 16. Subsequent Events

Effective January 1, 2019, the Company appointed Andrew Sassine as its Chief Financial Officer, whose employment terms are pending approval from the shareholders under the Israeli law. All compensation paid to Mr. Sassine from January 1, 2019 to the date of this filing is subject to refund to the Company should his appointment not be approved by shareholders.

On February 11, 2019, the Company announced its intention to initiate a process to redomicile from an Israeli limited company to a U.S. corporation. The final form and timing of the redomiciliation has not yet been finalized and the redomiciliation is subject to the approval of the shareholders. On February 11, 2019, the Company filed an application with the Tel Aviv District Court to approve the convening of a general shareholders meeting of the Company for the approval of the redomiciliation pursuant to Sections 350 and 351 of the Companies Law.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation and Principles of Consolidation

Segment Information

Cash and Cash Equivalents

Cash equivalents are short-term highly liquid investments that are readily convertible to cash with original maturities of three months or less at the date of purchase.

Restricted Cash

Restricted cash

Statement of cash flows

	As of December 31,
(in thousands)	2018		2017
Cash and cash equivalents	$	36,709	$	24,965
Restricted cash		—		166
Non-current Restricted cash		107		107
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$	36,816	$	25,238

Short-term Bank Deposits

Short-term Investments

Fair Value Measurements

Accounts Receivable

Concentration of Credit Risk and Significant Customers

There was one customer that comprised 96% of the total accounts receivable balance at December 31, 2018 and one customer that comprised the total accounts receivable balance at December 31, 2017.

Intangible Assets Held for Sale and Equity Method Investment

Property and Equipment, net

Comprehensive Income/Loss

Revenue Recognition

Research and Development Costs, net

Share-Based Compensation

Ordinary Shares Valuation

Income Tax Expense

Net Loss per Share

No dividends were declared or paid during the reported periods.

Recently Issued Accounting Pronouncements

Recently Adopted Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Schedule of Cash and Cash Equivalents and Restricted Cash

	As of December 31,
(in thousands)	2018		2017
Cash and cash equivalents	$	36,709	$	24,965
Restricted cash		—		166
Non-current Restricted cash		107		107
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$	36,816	$	25,238

Collaboration Revenue (Tables)

12 Months Ended

Dec. 31, 2018

Collaboration Agreements [Abstract]

Summary of Collaboration Revenue

	Year Ended December 31,
(Dollars in thousands)	2018		2017
Collaboration Partner - Janssen	$	1,232	$	4,862
Collaboration Partner - Ultragenyx		6,794		5,639
Collaboration Partner - Takeda		1,137		1,403
Collaboration Partner - CureVac		4,427		—
Other		2,163		1,094
	$	15,753	$	12,998

Short-term Investments (Tables)

12 Months Ended

Dec. 31, 2018

Investments Debt And Equity Securities [Abstract]

Summary of Short-term Investments

There were no short-term investments as of December 31, 2018. The following is a summary of short-term investments at December 31, 2017

	December 31, 2017
(Dollars in thousands)	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair value
Certificates of deposit	$	1,462	$	—	$	—		$	1,462
Corporate debt securities		7,149		—		(3	)		7,146
Total	$	8,611	$	—	$	(3	)	$	8,608

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Schedule of Financial Assets Measured at Fair Value on a Recurring Basis

	December 31, 2017
	Fair value measurements using input type
	Level 1		Level 2		Level 3		Total
Cash equivalents	$	2,024	$	—	$	—	$	2,024
Certificates of deposit		—		1,462		—		1,462
Corporate debt securities		—		7,146		—		7,146
Total financial assets	$	2,024	$	8,608	$	—	$	10,632

- Definition

Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances.

+ References + Details

Reverse Merger with Alcobra Ltd. (Tables)

12 Months Ended

Dec. 31, 2018

Business Combinations [Abstract]

Summary of Fair Value of Assets and Liabilities Acquired

The following summarizes the estimated fair value of the assets and liabilities acquired at November 15, 2017, the date of the merger:

(in thousands)
Cash and cash equivalents	$	2,032
Restricted cash		179
Short-term investments		34,188
Prepaid and other assets		434
Property, plant and equipment – held for sale		29
Intangible asset-held for sale		590
Total assets acquired		37,452
Accounts payable and accrued expenses		(1,906	)
Net assets acquired	$	35,546

Balance Sheet Details (Tables)

12 Months Ended

Dec. 31, 2018

Balance Sheet Related Disclosures [Abstract]

Schedule of Accrued Liabilities

Accrued liabilities consisted of the following as of December 31, 2018 and December 31, 2017.

	December 31,
(in thousands)	2018		2017
Accrued compensation	$	974	$	1,812
Refundable fees received		2,259		—
Other accrued liabilities		674		981
Total	$	3,907	$	2,793

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Summary of Components of Property and Equipment

Property and equipment, net consisted of the following:

	December 31,
(in thousands)	2018			2017
Research equipment	$	2,711		$	1,620
Computers and software		200			97
Office equipment and furniture		527			255
Leasehold improvements		34			44
Total		3,472			2,016
Less accumulated depreciation and amortization		(1,497	)		(967	)
Property and equipment, net	$	1,975		$	1,049

Debt (Tables)

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Schedule of Maturities of Long-term Debt

Principal payments, including the final payment due at repayment, on the long-term debt are as follows as of December 31, 2018:

Year Ending December 31,
2019	$	—
2020		2,666,667
2021		4,000,000
2021		3,633,333
Total		10,300,000

Share-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Weighted Average Assumptions Used in Black -Scholes Valuation Model

The following table presents the weighted-average assumptions used in the Black-Scholes valuation model by the Company in calculating the fair value of share options granted:

	For the Year Ended December 31,
	2018			2017
Expected life (in years)		6.07			7.3
Expected volatility		73.3	%		76.4	%
Expected dividend yield		—	%		—	%
Risk-free interest rate		2.77	%		1.87	%
Grant date weighted average fair value	$	5.38		$	7.94

Summary of Share Option Activity

The following table summarizes the Company’s share option activity for the year ended December 31, 2018:

	Number of Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (in thousands)
Outstanding – December 31, 2017		344,055		$	8.70
Granted		1,046,931		$	8.13
Exercised		(62,224	)	$	5.34			$	218
Forfeited/cancelled		(139,329	)	$	16.92
Outstanding – December 31, 2018		1,189,433		$	7.41		9.11	$	301
Exercisable – December 31, 2018		280,782		$	5.77		7.53	$	255
Exercisable and expected to vest – December 31, 2018		1,189,433		$	7.41		9.11	$	301

Schedule of Share-based Compensation Expenses

Share-based compensation expenses included in the Company’s statements of operations and comprehensive loss for the years ended December 31, 2018 and 2017 were:

	For the Year Ended December 31,
(in thousands)	2018		2017
Research and development	$	566	$	38
General and administrative		693		2,132
Total	$	1,259	$	2,170

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Summary of Reconciliation of Loss Before Income Taxes for Domestic and Foreign Locations

A reconciliation of loss before income taxes for domestic and foreign locations for the years ended December 31, 2018 and 2017 is as follows:

	For the Year Ended December 31,
(In thousands)	2018			2017
United States	$	(21,604	)	$	(10,820	)
Foreign		(181	)		(81	)
Total loss before income taxes	$	(21,785	)	$	(10,901	)

Summary of Reconciliation of Income Tax Expense

A reconciliation of income tax expense for the years ended December 31, 2018 and 2017 is as follows (in millions):

	December 31,
	2018		2017
Beginning balance of unrecognized tax benefits	$	0.4	$	0.4
Settlement of prior period tax positions		—		—
Increase for prior period tax positions		—		—
Increase for current period tax positions		—		—
Ending balance of unrecognized tax benefits	$	0.4	$	0.4

Significant Components of Deferred Income Taxes

The significant components of deferred income taxes at December 31, 2018 and 2017:

	December 31,
(in thousands)	2018			2017
Deferred tax assets:
Net operating loss	$	8,399		$	25,101
Tax credits		35			35
Accrued liabilities		261			227
Deferred revenue		1,713			1,162
Depreciation and amortization		46			—
Share-based compensation		85			90
Total gross deferred tax assets		10,539			26,615
Deferred tax liabilities:
Depreciation and amortization		—			(96	)
Valuation allowance		(10,539	)		(26,519	)
Net deferred tax asset	$	—		$	—

Reconciliation of Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate

A reconciliation of the federal statutory income tax rate to the Company’s effective income tax rate is as follows:

	For the Year Ended December 31,
	2018			2017
Federal statutory income tax rate		21.0	%		34.0	%
State income taxes, net of federal benefit		5.3	%		4.4	%
Foreign rate differential		(1.3	%)		0.2	%
Share-based compensation		(0.2	%)		—	%
Tax Cuts and JOBS Act		—	%		(22.0	%)
Change in tax rate		—	%		(8.3	%)
Change in valuation allowance		(20.7	%)		1.8	%
Other		(3.0	%)		(1.8	%)
Permanent differences		(1.1	%)		(8.3	%)
Provision for income taxes		—	%		—	%

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Schedule of Future Minimum Payments Under Leases and Lease Commitments

Future minimum payments under leases and lease commitments with initial terms greater than one year were as follows at December 31, 2018 (in thousands):

2019	$	1,268
2020		1,277
2021		1,315
2022		1,350
2023		1,390
Thereafter		1,745
Total	$	8,345

Organization (Details Textual)			12 Months Ended
Organization (Details Textual)	Oct. 12, 2018 USD ($)	Nov. 15, 2017 ₪ / shares shares	Dec. 31, 2018 USD ($)	Oct. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 ₪ / shares
Organization And Significant Accounting Policies [Line Items]
Business merger date		Nov. 15, 2017
Par value per share \| ₪ / shares						₪ 0.07
Reverse split of ordinary shares			1-for-7
Reverse split of ordinary shares, ratio		0.1429	0.1429
Accumulated deficit			$ (44,874,000)		$ (23,089,000)
Loan and Security Agreement [Member] \| Term Loan [Member]
Organization And Significant Accounting Policies [Line Items]
Gross proceeds from long-term debt agreement	$ 10,000,000
Sales Agreement [Member] \| Leerink Partners LLC [Member] \| Maximum [Member]
Organization And Significant Accounting Policies [Line Items]
Aggregate amount of ordinary shares sold				$ 30,000,000
Alocobra Limited [Member]
Organization And Significant Accounting Policies [Line Items]
Stock issued during period, shares, acquisitions \| shares		6,631,712
Par value per share \| ₪ / shares		₪ 0.07

Summary of Significant Accounting Policies (Details Textual)	12 Months Ended
	Dec. 31, 2018 USD ($) Segment Customer shares	Dec. 31, 2017 USD ($) Customer shares
Summary Of Significant Accounting Policy [Line Items]
Number of Operating Segments \| Segment	1
Non-current restricted cash	$ 107,000	$ 107,000
Lease expiration year	2025
Restricted cash, current		166,000
Available-for-sale securities	$ 0
Allowance for Doubtful Accounts Receivable	$ 0	0
Equity method ownership percentage	30.00%
Share of losses for investee	$ 302,000
Impairment losses	$ 0	0
Share options expiration period	10 years
Vesting period	4 years
Vesting description	Share-based compensation is recognized over the requisite service period of the individual awards using the straight-line attribution method, which generally equals the vesting period. Share options have a ten-year life and generally vest 25% on the first anniversary of the grant and in 1/48th equal installments on each monthly anniversary thereafter, such that options are fully vested on the four-year anniversary of the date of grant.
Share-based awards granted \| shares	1,046,931
Tax Cuts and Jobs Act, accounting complete, date	Dec. 22, 2017
Accounting requirements	$ 0
Tax cuts and jobs act of 2017, remeasurement of deferred tax assets	2,400,000
Dividends declared	0	0
Dividends paid	0	$ 0
ASU 2016-02 [Member]
Summary Of Significant Accounting Policy [Line Items]
Operating lease assets	5,500,000
Operating lease liabilities	$ 5,500,000
Incremental borrowing rate	8.40%
Derecognition of deferred rent	$ 500,000
Non-employee [Member]
Summary Of Significant Accounting Policy [Line Items]
Share-based awards granted \| shares	0	0
First Year Anniversary of the Grant [Member]
Summary Of Significant Accounting Policy [Line Items]
Vesting percentage	25.00%
Minimum [Member]
Summary Of Significant Accounting Policy [Line Items]
Equity method ownership percentage	20.00%
Property and equipment, useful lives	3 years
Maximum [Member]
Summary Of Significant Accounting Policy [Line Items]
Property and equipment, useful lives	5 years
Credit Risk [Member] \| Accounts Receivable [Member]
Summary Of Significant Accounting Policy [Line Items]
Number of customers \| Customer	1	1
Concentration risk, percentage	96.00%
Credit Risk [Member] \| Revenue [Member]
Summary Of Significant Accounting Policy [Line Items]
Number of customers \| Customer	3	3
Concentration risk, percentage	80.00%	92.00%

Schedule of Cash and Cash Equivalents and Restricted Cash (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 36,709,000	$ 24,965,000
Restricted cash		166,000
Non-current Restricted cash	107,000	107,000
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$ 36,816,000	$ 25,238,000	$ 8,345,000

Collaboration Revenue (Details Textual)			12 Months Ended
Collaboration Revenue (Details Textual)	Feb. 11, 2019	Oct. 24, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue			$ 15,753,000	$ 12,998,000
Collaboration Partner - CureVac Entered into Co-Development Agreement [Member] \| Subsequent Event [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement termination date	Feb. 11, 2019
Percentage of global rights reassumed	100.00%
Percentage share of collaborative arrangement	50.00%
Collaboration Partner - CureVac [Member] \| Subsequent Event [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage share of collaborative arrangement	50.00%
Research Collaboration and Exclusive License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue			15,753,000	12,998,000
Research Collaboration and Exclusive License Agreement [Member] \| Collaboration Partner Janssen 2015 Agreements [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue				4,900,000
Number of collaboration agreements \| Agreement						2
Revenue earned and recognized upon amortization of the upfront payment			2,000
Labor and expense reimbursements revenue				4,400,000
Research Collaboration and Exclusive License Agreement [Member] \| Collaboration Partner Janssen 2017 Agreements [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue			1,200	100
Upfront payment received			7,700,000
Revenue recognition potential milestone revenue recognized			56,500
Upfront fee received			7,700,000
Deferred revenue			6,500,000	7,600,000
Research Collaboration and Exclusive License Agreement [Member] \| Collaboration Partner Janssen 2017 Agreements [Member] \| Minimum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option exercise revenue range per target				1,000,000
Research Collaboration and Exclusive License Agreement [Member] \| Collaboration Partner Janssen 2017 Agreements [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option exercise revenue range per target				5,000,000
Research Collaboration and Exclusive License Agreement [Member] \| Collaboration Partner - Ultragenyx [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue			6,794,000	5,639,000
Labor and expense reimbursements revenue			2,300,000	3,700,000
Revenue recognition potential milestone revenue recognized			69,500,000
Deferred revenue			$ 2,700,000	5,800,000
Revenue recognition, milestone			Ultragenyx will pay the Company a one-time milestone payment after the first optimized lead designation for the first product with respect of such development target.
Royalty payment term description			Ultragenyx will pay royalties as a percentage of net sales on a product-by-product and country-by-country basis during the applicable royalty term up to and including double digits.
Amortization period expected termination date.			Mar. 31, 2019
Revenue recognized as one time payment				2,000,000
Research Collaboration and Exclusive License Agreement [Member] \| Collaboration Partner - Ultragenyx [Member] \| Up-Front Fee [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front fee deferred and recognized			$ 10,000,000
Research Collaboration and Exclusive License Agreement [Member] \| Collaboration Partner - Ultragenyx [Member] \| Minimum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option exercise revenue range per target						$ 2,000,000
Research Collaboration and Exclusive License Agreement [Member] \| Collaboration Partner - Ultragenyx [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option exercise revenue range per target						$ 5,000,000
Research Collaboration and Exclusive License Agreement [Member] \| Collaboration Partner - Takeda [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue			1,137,000	1,403,000
Upfront fee received					$ 100,000
Revenue recognition research activities period					18 months
Regularly scheduled research funding payments					$ 3,700,000
Research Collaboration and Exclusive License Agreement [Member] \| Collaboration Partner - CureVac [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue			4,427,000
Research Collaboration and Exclusive License Agreement [Member] \| Foreign Countries [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue			5,000,000	1,000,000
mRNA Technology [Member] \| Collaboration Partner - CureVac Entered into Co-Development Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue			3,300,000
mRNA Technology [Member] \| Collaboration Partner - CureVac [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue			4,400,000
Labor and expense reimbursements revenue			3,800,000
Deferred revenue			4,400,000
Upfront license fees			$ 5,000
Development and option agreement date			Jan. 31, 2018
Research and Exclusive License Agreement [Member] \| Synthetic Genomics, Inc [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue		$ 2,200,000
Upfront fee received		$ 200,000
Labor reimbursement and sublicense revenue recognized			$ 1,400,000
Research and Exclusive License Agreement [Member] \| Synthetic Genomics, Inc [Member] \| Minimum [Member] \| LUNAR Product [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of all cash payments received		10.00%
Research and Exclusive License Agreement [Member] \| Synthetic Genomics, Inc [Member] \| Minimum [Member] \| Non-LUNAR Product [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of consideration received		5.00%
Research and Exclusive License Agreement [Member] \| Synthetic Genomics, Inc [Member] \| Maximum [Member] \| LUNAR Product [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of all cash payments received		20.00%
Research and Exclusive License Agreement [Member] \| Synthetic Genomics, Inc [Member] \| Maximum [Member] \| Non-LUNAR Product [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of consideration received		10.00%
Research Collaboration and License Agreement [Member] \| Providence Therapeutics, Inc [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue related to amortization of upfront payment		$ 600,000	$ 600,000	$ 1,000,000

Collaboration Revenue - Summary of Collaboration Revenues (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue	$ 15,753	$ 12,998
Research Collaboration and Exclusive License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue	15,753	12,998
Research Collaboration and Exclusive License Agreement [Member] \| Collaboration Partner - Janssen [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue	1,232	4,862
Research Collaboration and Exclusive License Agreement [Member] \| Collaboration Partner - Ultragenyx [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue	6,794	5,639
Research Collaboration and Exclusive License Agreement [Member] \| Collaboration Partner - Takeda [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue	1,137	1,403
Research Collaboration and Exclusive License Agreement [Member] \| Collaboration Partner - CureVac [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue	4,427
Research Collaboration and Exclusive License Agreement [Member] \| Other Collaboration [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue	$ 2,163	$ 1,094

Short-term Investments (Details Textual) - USD ($)	12 Months Ended
Short-term Investments (Details Textual) - USD ($)	Dec. 31, 2017	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Total amount short-term investments	$ 23,608,000
Short Term Deposit Weighted Average Interest Rate	1.60%
Short-term investment		$ 0
Bank Deposits [Member]
Schedule Of Available For Sale Securities [Line Items]
Total amount short-term investments	$ 15,000,000

Summary of Short-term Investments (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	$ 8,611
Gross unrealized losses	3
Fair value	8,608
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	1,462
Fair value	1,462
Corporate Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	7,149
Gross unrealized losses	3
Fair value	$ 7,146

Schedule of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring [Member] $ in Thousands	Dec. 31, 2017 USD ($)
Assets Fair Value Disclosure [Abstract]
Cash equivalents	$ 2,024
Certificates of deposit	1,462
Corporate debt securities	7,146
Total financial assets	10,632
Fair Value, Inputs, Level 1 [Member]
Assets Fair Value Disclosure [Abstract]
Cash equivalents	2,024
Total financial assets	2,024
Fair Value, Inputs, Level 2 [Member]
Assets Fair Value Disclosure [Abstract]
Certificates of deposit	1,462
Corporate debt securities	7,146
Total financial assets	$ 8,608

Reverse Merger with Alcobra Ltd. (Details 1) - USD ($) $ in Thousands	Dec. 31, 2017	Nov. 15, 2017
Business Acquisition [Line Items]
Intangible asset held for sale	$ 590
Alocobra Limited [Member]
Business Acquisition [Line Items]
Cash and cash equivalents		$ 2,032
Restricted cash		179
Short-term investments		34,188
Prepaid and other assets		434
Property, plant and equipment – held for sale		29
Intangible asset held for sale		590
Total assets acquired		37,452
Accounts payable and accrued expenses		(1,906)
Net assets acquired		$ 35,546

Reverse Merger with Alcobra Ltd. (Details Textual)	12 Months Ended
Reverse Merger with Alcobra Ltd. (Details Textual)	Dec. 31, 2018 USD ($) $ / shares shares
Business Acquisition [Line Items]
Equity method ownership percentage	30.00%
Amiservice [Member]
Business Acquisition [Line Items]
Equity method investment, description	Assets acquired in the merger included an intangible asset consisting of in-process research and development for proprietary drug technology called ADAIR. At the closing date of the reverse merger, the Company entered into an agreement with Amiservice to which the Company agreed to transfer certain intellectual property related to ADAIR in exchange for a minority equity stake in a company to be formed by Amiservice for the purpose of acquiring the ADAIR assets. The Company determined that the asset met the classification criteria as held for sale in accordance with related accounting guidance when acquired and remained held for sale at December 31, 2017. To determine the fair value of the ADAIR asset, the Company utilized an independent valuation consultant who valued the asset using a market approach valuation method. In conjunction with this valuation, management judgment was required to forecast the occurrence of future events that would trigger the closing of the ADAIR sale agreement.
Alocobra Limited [Member]
Business Acquisition [Line Items]
Total fair value of consideration paid	$ 40,841,000
Shares outstanding as on merger date \| shares	3,997,000
Closing price as on merger date \| $ / shares	$ 10.22
Charge to additional paid in capital	$ 5,295,000
Payments for merger related costs	$ 1,734,000

Balance Sheet Details - Schedule of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Liabilities Current [Abstract]
Accrued compensation	$ 974	$ 1,812
Refundable fees received	2,259
Other accrued liabilities	674	981
Total	$ 3,907	$ 2,793

Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 3,472	$ 2,016
Less accumulated depreciation and amortization	(1,497)	(967)
Property and equipment, net	1,975	1,049
Research equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,711	1,620
Computer and software [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	200	97
Office equipment and furniture [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	527	255
Leasehold improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 34	$ 44

Debt (Details Textual) - USD ($)			12 Months Ended
Debt (Details Textual) - USD ($)	Oct. 12, 2018	Nov. 15, 2017	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Outstanding long-term debt			$ 9,911,000
Conversion of notes to ordinary shares				$ 5,957,000
Interest expense related to long-term debt			$ 186,000	$ 150,000
Convertible Promissory Notes [Member]
Debt Instrument [Line Items]
Convertible promissory note, amount converted		$ 5,795,000
Accrued interest		162,000
Conversion of notes to ordinary shares		$ 616,824
Common stock average conversion rate		$ 10.19
Recognized additional net expense for conversion		$ 348,000
Loan and Security Agreement [Member] \| Western Alliance Bank [Member] \| Long-term Debt [Member]
Debt Instrument [Line Items]
Gross proceeds from long-term debt agreement	$ 10,000,000
Loan maturity date	Oct. 01, 2022
Interest of the prime rate plus	1.25%
Loan interest payment terms			The loan has an interest-only period of 19 months, which could be extended by an additional 6 months if certain conditions are met, followed by an amortization period of 30 months, or 24 months if the interest-only period is extended.
Loan interest-only payment period	19 months		19 months
Loan interest-only payment period	6 months
Debt Instrument, amortization period	30 months		30 months
Debt Instrument, amortization period	24 months
Loan orgination fee paid	$ 128,000
Warrant fee payable	$ 350,000
Prepayment fee percentage	3.00%
Prepayment fee percentage	2.00%
Percentage of fee liable on principal balance	3.00%
Fee payable upon repayment	$ 300,000
Debt instrument, collateral			The Loan is collateralized by all of the assets of the Company, excluding intellectual property, which is subject to a negative pledge. The Loan contains customary conditions of borrowing, events of default and covenants, including covenants that restrict the Company’s ability to dispose of assets, merge with or acquire other entities, incur indebtedness and make distributions to holders of the Company’s capital stock. In addition, the Company is required to maintain at least 50% of its deposit and investment accounts, or $20 million, whichever is lower, with the Western Alliance Bank.
Debt instrument, collateral amount	20,000,000
Available sale of equity securities	$ 15,000,000
Outstanding long-term debt			$ 9,900,000
Accretion of final payment			28,000
Unamortized debt discount			$ 117,000
Loan and Security Agreement [Member] \| Western Alliance Bank [Member] \| Long-term Debt [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Percentage required to be maintain in deposit and investment	50.00%

Debt - Summary of Final Payment Due at Repayment (Details)	Dec. 31, 2018 USD ($)
Debt Disclosure [Abstract]
2020	$ 2,666,667
2021	4,000,000
2021	3,633,333
Total	$ 10,300,000

Shareholders' Equity (Details Textual)		1 Months Ended	12 Months Ended
Shareholders' Equity (Details Textual)	Nov. 15, 2017 shares	Mar. 31, 2013 $ / shares shares	Dec. 31, 2018 shares	Dec. 31, 2017 USD ($) Milestone shares	Dec. 31, 2016 Milestone shares	Dec. 31, 2014 shares
Class Of Stock [Line Items]
Reverse split of ordinary shares			1-for-7
Reverse split of ordinary shares, ratio	0.1429		0.1429
Common stock, shares authorized	30,000,000		30,000,000	30,000,000
Antidilutive securities excluded from computation of earnings per share			94,000	3,057,000
Treasury shares			43,000	43,000
Unvested Restricted Ordinary Shares [Member]
Class Of Stock [Line Items]
Antidilutive securities excluded from computation of earnings per share			622,667	622,667
Exercise of Share Options and Subject to Future Vesting [Member]
Class Of Stock [Line Items]
Antidilutive securities excluded from computation of earnings per share				79,000
Restricted Common Stock [Member]
Class Of Stock [Line Items]
Stock issued during period		2,783,686
Share price of stock issued \| $ / shares		$ 0.0068
Shares subject to repurchase option		1,538,353
Share based compensation arrangement by share based payment award period from termination date of employee or consultant for repurchase of shares		2 months
Number of milestones achieved \| Milestone				2	2
Common shares, unvested			622,667,000	622,667,000		769,176,000
Accelerated ordinary shares vested				146,510,000
Modification expense resulting from accelerated vesting of ordinary shares \| $				$ 1,495,000
Vested After Obtaining Suitable siRNA License [Member]
Class Of Stock [Line Items]
Shares vesting percentage			25.00%
Vested After Obtaining Suitable siRNA License [Member] \| Restricted Common Stock [Member]
Class Of Stock [Line Items]
Shares vesting percentage		25.00%
Vested After in Vivo Proof-of-concept Achieved [Member] \| Restricted Common Stock [Member]
Class Of Stock [Line Items]
Shares vesting percentage		25.00%
Vested After IND Application Completed [Member] \| Restricted Common Stock [Member]
Class Of Stock [Line Items]
Shares vesting percentage		25.00%
Vested After in Human Efficacy Achieved [Member] \| Restricted Common Stock [Member]
Class Of Stock [Line Items]
Shares vesting percentage		25.00%
Ordinary Shares [Member] \| Arcturus Inc [Member]
Class Of Stock [Line Items]
Stock conversion ratio	0.293
Warrant [Member]
Class Of Stock [Line Items]
Warrant outstanding					192,647
Common shares issued upon warrant conversion				188,980
Proceeds from warrant exercised \| $				$ 160,000

Share-Based Compensation (Details Textual)		1 Months Ended	12 Months Ended
Share-Based Compensation (Details Textual)	Nov. 15, 2017	Aug. 31, 2018 shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) BoardMember shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share options expiration period			10 years
Reverse share split, description			1-for-7
Reverse split of ordinary shares, ratio	0.1429		0.1429
Compensation cost not yet recognized \| $			$ 4,400,000
Weighted-average remaining service period			3 years 1 month 6 days
Fair value of options vested \| $			$ 972,000	$ 669,000
Exercisable and expected to vest options, subject to repuchase conditions			0	35,595
Share based compensation expense \| $			$ 1,259,000	$ 2,170,000
Number of Shares, Granted			1,046,931
General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation expense \| $			$ 693,000	$ 2,132,000
Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of board members \| BoardMember				2
Restricted Common Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation expense \| $				$ 1,495,000
Performance Based Options \| General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation expense \| $				$ 568,000
Performance Based Options \| Board of Members [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Granted				58,600
2013 Equity Incentive Plan [Member] \| Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized for grant			0
Share options expiration period			10 years
Common stock reserved for future issuance			38,751
2010 Equity Incentive Plan [Member] \| Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share options expiration period			10 years
Common stock reserved for future issuance			94,001
2018 Omnibus Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized for grant		1,100,000
Common stock reserved for future issuance			470,000
Shares issued in period		220,000
2018 Omnibus Equity Incentive Plan [Member] \| Non-Executive Officer [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued in period		130,000

Share-Based Compensation (Details) - $ / shares	12 Months Ended
Share-Based Compensation (Details) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant date weighted average fair value	$ 5.38	$ 7.94
Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected life (in years)	6 years 25 days	7 years 3 months 18 days
Expected volatility	73.30%	76.40%
Risk-free interest rate	2.77%	1.87%

Share-Based Compensation (Details 1) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares, Outstanding \| shares	344,055
Number of Shares, Granted \| shares	1,046,931
Number of Shares, Exercised \| shares	(62,224)
Number of Shares, Forfeited/cancelled \| shares	(139,329)
Number of Shares, Outstanding \| shares	1,189,433
Number of Shares, Exercisable – December 31, 2018 \| shares	280,782
Number of Shares, Exercisable and expected to vest – December 31, 2018 \| shares	1,189,433
Weighted-Average Exercise Price, Outstanding \| $ / shares	$ 8.70
Weighted-Average Exercise Price, Granted \| $ / shares	8.13
Weighted-Average Exercise Price, Exercised \| $ / shares	5.34
Weighted-Average Exercise Price, Forfeited/cancelled \| $ / shares	16.92
Weighted-Average Remaining Contractual Term (Years), Outstanding - December 31, 2018 \| $ / shares	7.41
Weighted-Average Remaining Contractual Term (Years), Exercisable - December 31, 2018 \| $ / shares	5.77
Weighted-Average Remaining Contractual Term (Years), Exercisable and expected to vest – December 31, 2018 \| $ / shares	$ 7.41
Weighted-Average Remaining Contractual Term (Years), Outstanding - December 31, 2018	9 years 1 month 9 days
Weighted-Average Remaining Contractual Term (Years), Exercisable - December 31, 2018	7 years 6 months 10 days
Weighted-Average Remaining Contractual Term (Years), Exercisable and expected to vest – December 31, 2018	9 years 1 month 9 days
Aggregate Intrinsic Value, Exercised \| $	$ 218
Aggregate Intrinsic Value, Outstanding – December 31, 2018 \| $	301
Aggregate Intrinsic Value, Exercisable – December 31, 2018 \| $	255
Aggregate Intrinsic Value, Exercisable and expected to vest – December 31, 2018 \| $	$ 301

Share-Based Compensation (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expenses	$ 1,259	$ 2,170
Research and Development [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expenses	566	38
General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expenses	$ 693	$ 2,132

Income Taxes (Details 1) - USD ($) $ in Thousands	12 Months Ended
Income Taxes (Details 1) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
United States	$ (21,604)	$ (10,820)
Foreign	(181)	(81)
Total loss before income taxes	$ (21,785)	$ (10,901)

Income Taxes (Details Textual) - USD ($)		12 Months Ended
Income Taxes (Details Textual) - USD ($)	Jan. 01, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Maximum percentage of likelihood of uncertain tax position recognized to be sustained		50.00%
Unrecognized Tax Benefits		$ 400,000	$ 400,000	$ 400,000
Unrecognized tax benefits impacting effective tax rate		0	0
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense		0	0
Research and Development Expenses		$ 16,982,000	$ 15,918,000
Net Operating Loss and Research and Development Credit Carryforwards Limitations On Cumulative Change In Ownership Minimum Percentage		50.00%
Net Operating Loss and Research and Development Credit Carryforwards Limitations on Cumulative Change In Ownership Period		3 years
Federal statutory income tax rate	21.00%	21.00%	34.00%
Measurement period adjustment		$ 0
Provisional amount offsetted by valuation allowance			$ 2,400,000
Maximum [Member]
Income Taxes [Line Items]
Federal statutory income tax rate			35.00%
Federal [Member]
Income Taxes [Line Items]
Operating Loss Carryforwards		$ 33,200,000
Operating Loss Carryforwards Expiration Year		2034
Federal [Member] \| Research Tax Credit Carryforward [Member]
Income Taxes [Line Items]
Credit Carryforward Amount		$ 200,000
Credit Carryforward Expiration Year		2033
Federal [Member] \| Alcobra Inc. and Alcobra Ltd. [Member]
Income Taxes [Line Items]
Net operating loss amount excluded from deferred tax assets		$ 300,000
State [Member]
Income Taxes [Line Items]
Operating Loss Carryforwards		$ 30,200,000
Operating Loss Carryforwards Expiration Year		2034
State [Member] \| Research Tax Credit Carryforward [Member]
Income Taxes [Line Items]
Credit Carryforward Amount		$ 200,000
Credit Carryforward Expiration Year		2033
Foreign [Member]
Income Taxes [Line Items]
Operating Loss Carryforwards		$ 89,000,000
Foreign [Member] \| Alcobra Inc. and Alcobra Ltd. [Member]
Income Taxes [Line Items]
Net operating loss amount excluded from deferred tax assets		20,400,000
United States [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits		200,000
California [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits		$ 200,000

Income Taxes (Details 3) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss	$ 8,399	$ 25,101
Tax credits	35	35
Accrued liabilities	261	227
Deferred revenue	1,713	1,162
Depreciation and amortization	46	0
Share-based compensation	85	90
Total gross deferred tax assets	10,539	26,615
Deferred tax liabilities:
Depreciation and amortization		(96)
Valuation allowance	$ (10,539)	(26,519)
Net deferred tax asset		$ 0

Income Taxes (Details 4)		12 Months Ended
Income Taxes (Details 4)	Jan. 01, 2018	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	21.00%	34.00%
State income taxes, net of federal benefit		5.30%	4.40%
Foreign rate differential		(1.30%)	0.20%
Share-based compensation		(0.20%)
Tax Cuts and JOBS Act			(22.00%)
Change in tax rate			(8.30%)
Change in valuation allowance		(20.70%)	1.80%
Other		(3.00%)	(1.80%)
Permanent differences		(1.10%)	(8.30%)

Commitments and Contingencies (Details Textual) - USD ($)	1 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Commitment And Contingencies [Line Items]
Operating lease rent expense		$ 1,100,000	$ 334,000
October 2017 Lease Amendment [Member]
Commitment And Contingencies [Line Items]
Operating lease extended additional term	84 months
Lessor, operating lease, term of contract	4 months
Tenant improvement allowance	$ 74,000
Lessor leasing arrangements operating leases extended lease	15 years
Security	$ 96,000
San Diego, California [Member]
Commitment And Contingencies [Line Items]
Operating lease expiration period year and month		2018-02
Cystic Fibrosis Foundation Therapeutics Grants [Member]
Commitment And Contingencies [Line Items]
Grants received		$ 500,000

Commitments and Contingencies (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 1,268
2020	1,277
2021	1,315
2022	1,350
2023	1,390
Thereafter	1,745
Total	$ 8,345

Related Party Transactions (Details Textual) - USD ($)		1 Months Ended	3 Months Ended	12 Months Ended
Related Party Transactions (Details Textual) - USD ($)	Oct. 24, 2017	May 31, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Stock compensation expense recognized				$ 1,259,000	$ 2,170,000
Professional fees		$ 1,400,000
Providence Therapeutics, Inc [Member] \| Research Collaboration and License Agreement [Member]
Related Party Transaction [Line Items]
Revenue related to amortization of upfront payment	$ 600,000			600,000	1,000,000
Accounts receivable, outstanding				$ 0	$ 0
Stock compensation expense recognized			$ 1,500,000

Income Taxes (Details 2) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Reconciliation Of Unrecognized Tax Benefits Excluding Amounts Pertaining To Examined Tax Returns Roll Forward
Beginning balance of unrecognized tax benefits	$ 0.4	$ 0.4
Ending balance of unrecognized tax benefits	$ 0.4	$ 0.4