Nightstar Therapeutics (NITE) 10-K 18 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Mar. 01, 2019	Jun. 29, 2018
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	NITE
Entity Registrant Name	Nightstar Therapeutics plc
Entity Central Index Key	0001711675
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	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Public Float			$ 149,156,960
Entity Common Stock, Shares Outstanding		33,483,821

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 27,784	$ 129,404
Marketable securities	136,385	0
Research and development credit	8,268	1,753
Prepaid expenses and other current assets	6,605	3,685
Total current assets	179,042	134,842
Property and equipment, net	562	355
Other assets	409
Total assets	180,013	135,197
Current liabilities:
Accounts payable	3,017	3,196
Accrued expenses and other liabilities	11,360	6,189
Total current liabilities	14,377	9,385
Total liabilities	14,377	9,385
Commitments and contingencies (see Note 14)
Shareholders’ equity:
Ordinary shares, £0.01 nominal value; 100,000 shares authorized as of December 31, 2018 and 2017; 33,486 and 28,904 shares issued and outstanding at December 31, 2018 and 2017, respectively	61	1
Additional paid-in capital	267,233	185,943
Accumulated other comprehensive income (loss):
Cumulative foreign currency translation adjustment	(4,855)	1,890
Unrealized holding gains on available for sales securities	2,081
Total accumulated other comprehensive income (loss)	(2,774)	1,890
Accumulated deficit	(98,884)	(62,022)
Total shareholders’ equity	165,636	125,812
Total liabilities and shareholders’ equity	$ 180,013	$ 135,197

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - £ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Ordinary shares, nominal value	£ 0.01	£ 0.01
Ordinary shares, authorized	100,000,000	100,000,000
Ordinary shares, issued	33,485,866	28,904,714
Ordinary shares, outstanding	33,485,866	28,904,714

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses:
Research and development	$ 31,585,000	$ 20,502,000	$ 10,165,000
General and administrative	12,572,000	7,001,000	2,055,000
Total operating expenses	44,157,000	27,503,000	12,220,000
Other income (expense):
Interest and other income	2,862,000	709,000	22,000
Other income (expense), net	3,993,000	(2,855,000)
Total other income (expense), net	6,855,000	(2,146,000)	22,000
Loss before (benefit from) provision for income taxes	(37,302,000)	(29,649,000)	(12,198,000)
(Benefit from) provision for income taxes	(440,000)	37,000	0
Net loss	(36,862,000)	(29,686,000)	(12,198,000)
Other comprehensive income (loss):
Foreign currency translation adjustment	(6,745,000)	3,988,000	(1,385,000)
Unrealized holding gains on available for sale securities	2,081,000
Total other comprehensive loss	$ (41,526,000)	$ (25,698,000)	$ (13,583,000)
Basic and diluted net loss per ordinary share	$ (1.26)	$ (1.63)	$ (1.36)
Weighted average basic and diluted ordinary shares	29,176,000	18,187,000	8,954,000

Consolidated Statements of Shar

Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Ordinary Shares	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at Dec. 31, 2015	$ 11,102		$ 31,953	$ (713)	$ (20,138)
Balance, shares at Dec. 31, 2015		8,694
Issuance of ordinary shares	11,317		11,317
Issuance of ordinary shares, shares		3,335
Issuance of restricted share awards, net, shares		516
Share-based compensation expense	208		208
Foreign currency translation adjustment	(1,385)			(1,385)
Net loss	(12,198)				(12,198)
Balance at Dec. 31, 2016	9,044		43,478	(2,098)	(32,336)
Balance, shares at Dec. 31, 2016		12,545
Issuance of ordinary shares	141,093	$ 1	141,092
Issuance of ordinary shares, shares		15,898
Issuance of restricted share awards, net, shares		461
Share-based compensation expense	1,373		1,373
Foreign currency translation adjustment	3,988			3,988
Net loss	(29,686)				(29,686)
Balance at Dec. 31, 2017	125,812	$ 1	185,943	1,890	(62,022)
Balance, shares at Dec. 31, 2017		28,904
Issuance of ordinary shares	77,169	$ 60	77,109
Issuance of ordinary shares, shares		4,600
Cancellation of restricted share awards, shares		(47)
Vesting of restricted share units, shares		29
Share-based compensation expense	4,181		4,181
Foreign currency translation adjustment	(6,745)			(6,745)
Unrealized holding gains on available for sale securities	2,081			2,081
Net loss	(36,862)				(36,862)
Balance at Dec. 31, 2018	$ 165,636	$ 61	$ 267,233	$ (2,774)	$ (98,884)
Balance, shares at Dec. 31, 2018		33,486

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (36,862)	$ (29,686)	$ (12,198)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	276	206	138
Non-cash share-based compensation	4,181	1,373	208
Accretion on marketable securities	(1,136)
Deferred taxes	(408)	(15)
Unrealized foreign exchange gains	(389)
Realized investment gains	(6,741)
Changes in operating assets and liabilities:
Research and development tax credit receivable	(6,921)	1,320	(849)
Prepaid expenses and other assets	(3,256)	(2,228)	(794)
Accounts payable	(9)	1,773	419
Accrued expenses and other liabilities	5,633	1,446	2,959
Net cash used in operating activities	(45,632)	(25,796)	(10,117)
Cash flows from investing activities:
Investments marketable securities	(205,290)
Proceeds from maturities of marketable securities	70,000
Purchases of property and equipment	(709)	(166)	(306)
Net cash used in investing activities	(135,999)	(166)	(306)
Cash flows from financing activities:
Proceeds of issuance of ordinary shares, net of issuance costs	77,164	141,093	11,317
Net cash provided by financing activities	77,164	141,093	11,317
Effect of exchange rate changes on cash, cash equivalents and restricted cash	2,905	4,151	(1,448)
Net increase (decrease) in cash, cash equivalents and restricted cash	(101,562)	119,282	(554)
Cash, cash equivalents and restricted cash, beginning of period	129,404	10,122	10,676
Cash, cash equivalents and restricted cash, end of period	27,842	$ 129,404	$ 10,122
Supplemental disclosure of cash flow information:
Cash paid for income taxes	$ 71

Nature of the Business

Nature of the Business	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business	1. Nature of the Business Nightstar Therapeutics plc (the “Company”) is a clinical-stage gene therapy company focused on developing and commercializing novel one-time treatments for patients suffering from rare inherited retinal diseases that would otherwise progress to blindness. The Company is developing a pipeline of proprietary product candidates that are designed to substantially modify or halt the progression of inherited retinal diseases for which there are no currently approved treatments. The Company’s lead product candidate, NSR-REP1, for the treatment of choroideremia (“CHM”) is in Phase 3 clinical development. The Company’s second product candidate, NSR-RPGR, is in Phase 2/3 clinical development for the treatment of X-linked retinitis pigmentosa. The Company also has product candidates in preclinical development for a number of inherited retinal diseases for which there are no approved treatments such as Stargardt disease. The Company is a public limited company incorporated in England and Wales. On October 2, 2017 the Company completed its initial public offering (“IPO”) of American Depositary Shares (“ADSs”). In the IPO, the Company sold an aggregate of 6,164,000 ADSs representing the same number of ordinary shares at a public offering price of $14.00 per ADS. Net proceeds were approximately $77.4 million, after deducting underwriting discounts and commissions and offering expenses paid by the Company. As described in Note 8, in October 2018, the Company completed a follow-on public offering of 4,600,000 ADSs resulting in net proceeds of approximately $77.2 million. Prior to its IPO in October 2017, the Company historically conducted its business through NightstaRx Limited (“NSL”) and its U.S. subsidiary, Nightstar, Inc. (“NSI”) and therefore the historical consolidated financial statements previously presented the consolidated results of operations of NSL. Following the completion of the Company’s IPO in October 2017, the consolidated financial statements present the consolidated results of operations of Nightstar Therapeutics plc. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, the timelines for clinical development and the results of our clinical trials, compliance with government regulations, receipt of regulatory approval for the Company’s product candidates, uncertainty of market acceptance of the Company’s products, securing reimbursement from government or third-party payors, procuring adequate supplies of the Company’s products and product candidates that comply with current good manufacturing practices and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of capital, adequate personnel and infrastructure, and extensive compliance-reporting capabilities. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize revenue from its product sales. The Company has funded its operations primarily from the sale of its ADSs and ordinary shares. The Company has incurred recurring losses and has had an accumulated deficit since its inception. The Company expects to continue to generate operating losses for the foreseeable future. The Company believes the cash, cash equivalents and marketable securities on hand will be sufficient to fund the Company’s operations for at least 12 months from the date of this Annual Report on Form 10-K. The future viability of the Company beyond that point is dependent on its ability to raise additional capital to finance its operations. The Company’s inability to raise capital as and when needed could have a negative impact on its financial condition and ability to pursue its business strategies. There can be no assurances, however, that the current operating plan will be achieved or that additional funding will be available on terms acceptable to the Company, or at all.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation These consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (U.S. GAAP). The consolidated financial statements include the accounts of the Company and all subsidiaries. All intercompany accounts and transactions have been eliminated. Research and development tax credit receivable was previously included in prepaid and other assets at December 31, 2017 and has been presented as a separate line item in the consolidated financial statements and related footnotes, to conform with the current year presentation. Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of income and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual for research and development expenses, the determination of the fair value of share-based awards issued, share-based compensation expense and the recoverability of the Company’s net deferred tax assets and related valuation allowance. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ materially from those estimates. Cash and Cash Equivalents The Company considers all highly liquid investments that have maturities of three months or less when acquired to be cash equivalents. Cash equivalents consist of investments in money market funds and United States (U.S.) Treasury securities with an original maturity of less than three months. Restricted Cash The Company has entered into a certain lease transaction (refer to Note 14) that requires a security deposit of $58,000 for the duration of the lease contract, which is reported in the Other assets balance line on the consolidated balance sheet. The Company includes the restricted cash balance in cash and cash equivalents when reconciling beginning-of-period and end-of-period total amounts shown on the consolidated statements of cash flows. Marketable securities The Company invests available funds in high-quality Treasury securities that are classified as available-for-sale and carried at fair value. Changes in fair value of available-for-sale securities are recorded in other comprehensive income (loss) as net unrealized gains (losses) on available-for-sale securities. The Company recognized $2.1 million (net of a tax benefit of $0.4 million) in unrealized holding gains for the year ended December 31, 2018. The Company did not recognize any net unrealized gains or losses on marketable securities in any prior period. Fair Value Measurements Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: ▪ Level 1—Quoted prices in active markets for identical assets or liabilities. ▪ Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. ▪ Level 3—Unobservable inputs that are supported by little or no market activity that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques For marketable securities, exclusively U.S. Treasury securities, management utilizes values provided by the Company’s investment advisor and compares them to the values published by a third-party source. Management believes that the carrying amounts of the Company’s consolidated financial instruments, including cash equivalents, prepaid expenses, research and development credit receivables, accounts payable and accrued expenses approximate fair value due to the short-term nature of those instruments. Concentration of Credit Risk Financial instruments that subject the Company to credit risk consist primarily of cash, cash equivalents and marketable securities. The Company places cash and cash equivalents in established financial institutions. Marketable securities held by the Company consist exclusively of U.S. Treasury securities. The Company has no significant off-balance-sheet risk or concentration of credit risk, such as foreign exchange contracts, options contracts, or other foreign hedging arrangements. Property and Equipment Property and equipment are recorded at cost and depreciated using the straight-line method over the estimated useful lives of the respective assets. As of December 31, 2018 and 2017, the Company’s property and equipment consisted of lab equipment, computer equipment and office equipment, which has an estimated useful life of three years. The Company capitalizes the cost of leasehold improvements and amortizes them over the shorter of the useful life of the asset and the non-cancellable lease term. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the consolidated statement of operations and other comprehensive loss. Expenditures for repairs and maintenance are charged to expense as incurred. The Company evaluates assets for potential impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparing the book values of the assets to the expected future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book values of the assets exceed their fair value. The Company has not recognized any impairment losses from inception through December 31, 2018. Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker in deciding how to allocate resources and assess performance. The Company and the Company’s chief operating decision maker, the Company’s Chief Executive Officer, view the Company’s operations and manage its business as a single operating segment, which is the business of developing and commercializing gene therapies; however, the Company operates in two geographic regions: United Kingdom and United States. Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries, share-based compensation and benefits, depreciation expense, travel, third-party license fees, and external costs of outside vendors engaged to conduct clinical development activities, clinical trials, cost to manufacture clinical trial materials and tax credits associated with research and development activities. Research and development tax credits received from Her Majesty's Revenue & Customs (“HMRC”) are recognized as offsets to research and development expenses. Research Contract Costs and Accruals The Company has entered into various research and development-related contracts with research institutions and other companies. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies or clinical trials, including the phase or completion of events, invoices received and contracted costs. Estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs. Patent Costs The Company expenses patent application and related legal costs as incurred and classified such costs as general and administrative expenses in the accompanying consolidated statements of operations and comprehensive loss. Share-Based Compensation The Company recognizes compensation expense for equity awards based on the grant date fair value of the award on a straight-line basis over the requisite service period. The Company uses the fair value of its ordinary shares to determine the fair value of restricted share awards (“RSAs”) and restricted share units (“RSUs”). The fair value of options is determined using the Black-Scholes option pricing model. The Company accounts for forfeitures as they occur. Foreign Currency Translation The Company maintains its financial statements in the functional currency pounds sterling. Monetary assets and liabilities denominated in currencies other than the functional currency are remeasured into the functional currency at rates of exchange prevailing at the balance sheet dates. Non-monetary assets and liabilities denominated in foreign currencies are remeasured into the functional currency at the exchange rates prevailing at the date of the transaction. Exchange gains or losses arising from foreign currency transactions are included in the determination of net loss for the respective periods. The Company recorded foreign exchange losses of $2.8 million, $2.9 million and $36,000 for the years ended December 31, 2018, 2017 and 2016, respectively. For financial reporting purposes, the financial statements of the Company, which are prepared using the functional currency of pounds sterling, have been translated into the U.S. dollar. Assets and liabilities are translated at the exchange rates at the balance sheet dates, revenue and expenses are translated at the average exchange rates and shareholders’ equity is translated based on historical exchange rates. Translation adjustments are not included in the determination of net loss, but are included as foreign currency adjustments in accumulated other comprehensive income (loss), a component of shareholders’ equity. The Company recorded a loss of $6.7 million, a gain of $4.0 million, and a loss of $1.4 million on foreign currency translation adjustments for the years ended December 31, 2018, 2017 and 2016, respectively. Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in its tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the consolidated financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that deferred tax assets will be recovered in the future to the extent management believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed as the amount of benefit to recognize in the consolidated financial statements. The amount of benefits that may be used is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate, as well as the related net interest and penalties. The Company recognizes interest and penalties related to unrecognized tax benefits on the income tax expense line in the accompanying consolidated statement of operations. As of December 31, 2018, 2017 and 2016, no accrued interest or penalties are included on the related tax liability line in the consolidated balance sheet. Research and development tax credits received from HMRC are recognized as offsets to research and development expenses. Comprehensive Loss The Company follows the provisions of the Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) Topic 220, Comprehensive Income Net Loss per Share Basic and diluted net loss per ordinary share is determined by dividing net loss by the weighted average number of ordinary shares outstanding during the period. For all periods presented, the outstanding shares of unvested RSAs and other common share equivalents were excluded from the calculation because the effect would be anti-dilutive. Therefore, the weighted average shares outstanding used to calculate both basic and diluted loss per share are the same. Smaller Reporting Company Status In June 2018 the U.S Securities and Exchange Commission amended the thresholds for the smaller reporting company (“SRC”) definition thereby expanding the number of smaller companies eligible to comply with the scaled disclosure requirements. As of December 31, 2018, the Company qualifies for these reduced SRC disclosure requirements. The Company will maintain the SRC status until its revenue and unaffiliated market capitalization exceed the eligibility criteria. Emerging Growth Company Status As of December 31, 2018, the Company maintained its status as an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act. The Company as an “emerging growth company, may take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not emerging growth companies. The Company may take advantage of these exemptions until the Company is no longer an emerging growth company. The Company may take advantage of these exemptions up until the last day of the fiscal year following the fifth anniversary of its initial public offering that occurred in October 2017, or such earlier time, that it ceases to be an emerging growth company. Recent Accounting Pronouncements Recently issued accounting pronouncements not yet adopted In February 2016, the FASB issued Auditing Standard Update (“ASU”) 2016-02, Leases. Codification Improvements to Topic 842, Leases Targeted Improvements , In August 2018, the FASB issued ASU 2018-13 ( Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement , reducing certain disclosures concerning the fair value hierarchy. T In ASU 2018-09 (“ASU 2018-09”), Codification Improvements . The ASU 2018-09 provides updates and clarifications to a number of previously issued accountings standards and such updates and clarifications are effective concurrently with the related standards. The Company does not expect the adoption of ASU 2018-09 to In June 2018, the FASB issued ASU 2018-07 , Compensation—Stock Compensation (Topic 718) - Improvements to Nonemployee Share-Based Payment Accounting Revenue from contracts with customers. In June 2016, the FASB issued ASU 2016-13, Financial Instruments Credit Losses (Topic 326) The Company has considered other recent accounting pronouncements and concluded that they are either not applicable to the business, or that the effect is not expected to be material to the consolidated financial statements as a result of future adoption. Recently adopted accounting pronouncements In October 2016, the FASB issued ASU No. 2016-16, Income Taxes (Topic 740) In March 2016, the FASB issued ASU No. 2016-09, Compensation—Stock Compensation (Topic 718 : Improvements to Employee Share-Based Payment Accounting In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash In January 2017, the FASB issued ASU 2017-01, Clarifying the Definition of a Business On August 26, 2016, the FASB issued Accounting Standards Update No 2016-15, Statement of Cash Flows (Topic 230) In January 2016, the FASB issued ASU 2016-01, Financial Instruments – Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities In August 2018, the FASB issued A ccounting Standards Update (“ASU”) , Intangibles – Goodwill and Other – Internal Use Software (Subtopic 350-40), Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract The guidance is effective for the Company in annual periods beginning after December 15, 2019, and interim periods within those annual periods. The Company has the option to apply the guidance prospectively to all implementation costs incurred after the date of adoption or retrospectively. The new guidance requires certain disclosures in the interim and annual period of adoption. On October 1, 2018, the Company adopted ASU 2018-15 on a prospective basis. The adoption did not have a material impact to the consolidated financial statements.

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair value measurements	3. Fair Value of Financial Assets and Liabilities The Company’s investments are all classified within Levels 1 and 2 of the fair value hierarchy. The Company’s investments classified within Level 1 of the fair value hierarchy are valued based quoted prices in active markets. The Company’s investments classified in Level 2 of the fair value hierarchy are based on matrix pricing compiled by third party pricing vendors, using observable market inputs such as interest rates, yield curves, and credit risk. For cash, current receivables, and accounts payable, the carrying amounts approximate fair value, because of the short maturity of these instruments, and therefore fair value information is not included in the table below (in thousands): December 31, 2018 Total Level 1 Level 2 Level 3 Money market funds and short-term U.S. Treasury securities, included in cash and cash equivalents $ 23,514 $ 23,514 $ — $ — U.S Treasury securities $ 136,385 100,949 35,436 — $ 159,899 $ 124,463 $ 35,436 $ — The Company did not have transfers in or out of Level 3 of the fair value hierarchy during the years ended December 31, 2017 and 2016. The Company had no liabilities measured at fair value on a recurring basis as of December 31, 2018. The Company had no assets or liabilities measured at fair value on a recurring basis as of December 31, 2017.

Marketable securities

Marketable securities	12 Months Ended
	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Marketable securities	4. Marketable securities As of December 31, 2018, the Company has the following investments in marketable securities classified as available-for-sale (in thousands): Maturity Amortized cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Impact of Foreign Currency Exchange Rate Fluctuations Aggregate Estimated Fair Value U.S. Treasury securities 3 - 9 months $ 133,899 $ — $ (21 ) $ 2,507 $ 136,385 All of the Company’s investments are classified as available-for-sale and are carried at fair value with unrealized gains and losses recorded as a component of accumulated other comprehensive income (loss), net of related income taxes. The Company-held investments of $159.9 million as of December 31, 2018 of which $136.4 million are classified as available-for-sale and $23.5 million are classified as cash equivalents. The Company recognized in accumulated other comprehensive income a $2.1 million (net of a tax benefit of $0.4 million) in net unrealized gains related to changes in the securities fair values, impacted by both rates and currency. No securities have been in an unrealized loss position for more than one year. During the year ended December 31, 2018, the Company realized $6.7 million in foreign currency gains on the settlement of maturing marketable securities as a result of favorable foreign currency exchange rate fluctuations. Realized gains and losses are determined using the specific identification method, reclassified from accumulated other comprehensive income and included in other income, net on the statement of comprehensive income. As of December 31, 2018, no securities are considered to be other than temporarily impaired because the impairments are not severe, have been for a short duration, and are due to normal market and interest rate fluctuations. Furthermore, the Company does not intend to sell the investment securities in an unrealized loss position and it is unlikely that the Company will be required to sell these securities before the recovery of the value.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2018
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid Expenses and Other Current Assets	5. Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following (in thousands): December 31, 2018 2017 Prepayments $ 5,236 $ 2,586 Research grant receivable 576 — Value-added tax receivable 526 905 Interest receivable 128 98 Other 139 96 Total prepaid expenses and other current assets $ 6,605 $ 3,685

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Property and Equipment, Net	6. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): December 31, 2018 2017 Lab equipment $ 358 $ 363 Computer and office equipment 554 382 Leasehold improvements 284 — Less: accumulated depreciation (634 ) (390 ) Total property and equipment, net $ 562 $ 355 Depreciation expense was $276,000, $206,000 and $138,000 for the years ended December 31, 2018, 2017 and 2016, respectively.

Accrued Expenses and Other Liab

Accrued Expenses and Other Liabilities	12 Months Ended
	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued Expenses and Other Liabilities	7. Accrued Expenses and Other Liabilities Accrued expenses and other liabilities consisted of the following (in thousands): December 31, 2018 2017 Research and development activities $ 6,527 $ 3,779 Compensation and benefits 2,423 1,093 Professional fees 636 410 Other 1,774 907 Total accrued expenses and other liabilities $ 11,360 $ 6,189

Shareholders' Equity

Shareholders' Equity	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Shareholders' Equity	8. Shareholders’ Equity Initial Public Offering and Subsequent offering On October 2, 2017, the Company closed its IPO of ADSs. In the IPO, the Company sold an aggregate of 6,164,000 at a public offering price of $14.00 per ADS, which included the exercise in full by the underwriters of their option to purchase additional ADSs. Net proceeds were approximately $77.4 million, after deducting underwriting discounts, and commissions and offering expenses paid by the Company of $2.9 million. Ordinary Share Conversion On the date of the IPO, the Company converted its outstanding ordinary shares. All share and per share information has been retroactively adjusted for this share conversion (See Notes 9 and 10). On October 2, 2018, the Company completed an underwritten follow-on public offering of 4.6 million ADSs, representing the same number of ordinary shares, at a public offering price of $18.00 per ADS, which included the exercise in full by the underwriters of their option to purchase additional ADSs. Net proceeds to the Company from the offering were approximately $77.2 million, after deducting underwriting discounts and commissions and offering expenses paid by the Company of $0.7 million. Corporate Reorganization On September 11, 2017, all shareholders of NightstaRx Limited exchanged each of the ordinary shares of different classes held by them for the same number and class of newly issued ordinary shares of Nightstar Therapeutics Limited and, as a result, NightstaRx became a wholly-owned subsidiary of Nightstar Therapeutics Limited on that date. On September 15, 2017, Nightstar Therapeutics Limited re-registered as a public limited company and was renamed Nightstar Therapeutics plc. On the date of the IPO, each separate class of ordinary shares of Nightstar Therapeutics plc was converted as stipulated in the articles of association into a single class of ordinary shares of Nightstar Therapeutics plc. The ratio for the conversion of each class of ordinary shares of Nightstar Therapeutics plc into ordinary shares of Nightstar Therapeutics plc was determined based on the IPO price. All share and per share information has been retroactively adjusted for this share conversion. Ordinary Shares Each holder of ordinary shares is entitled to one vote per ordinary share and to receive dividends when and if such dividends are recommended by the board of directors and declared by the shareholders. The Company has not declared any dividends since its inception. In addition to the issuance of Class A ordinary shares to founders and investors, the Company has issued Class B, C, D, E and F ordinary shares to certain employees as employee incentive awards, all of which were converted into a single class of ordinary shares as part of the corporate reorganization described above. March and June 2017 Financings In March 2017, the Company entered into a subscription and shareholders’ agreement, (the “March 2017 Agreement”), whereby certain shareholders elected to subscribe for an aggregate of 8,070,314 Class A ordinary shares as follows: 1,846,071 Class A ordinary shares at a price of £1.00 per share and 6,224,243 Class A ordinary shares at a price of £2.0737 per share. The Company issued all 8,070,314 Class A ordinary shares pursuant to the March 2017 Agreement on June 27, 2017 for gross proceeds of $18.8 million, and the Company incurred issuance costs of $50,000. The shares were issued to satisfy all of the outstanding tranche obligations, discussed below in this note under “Equity-based Tranche Obligation Liability.” The March 2017 Agreement was terminated in June 2017. In June 2017, the Company entered into a subscription and shareholders’ agreement, (the “June 2017 Agreement”), pursuant to which the Company issued an aggregate of 11,203,837 Class A ordinary shares at a price of $4.01648 per share for gross proceeds of $45.0 million, and the Company incurred issuance costs of $295,000. The June 2017 Agreement was terminated at the time of the Company’s corporate reorganization and IPO. Equity-based Tranche Obligation Liability The Company assessed the obligations to issue additional shares as part of the Amended 2013 Agreement and as part of the 2015 Agreement (collectively, the “Tranche Obligations”). The Company concluded that such obligations were freestanding financial instruments that were required to be separately recorded at fair value on the date the financing agreements were executed. With the exception of the shares that were to be issued upon satisfaction of the Tranche 3A and 3B milestones, contemplated by the Amended 2013 Agreement, all other Tranche Obligations were considered indexed to the Company’s ordinary shares, met all additional criteria in order to be classified as equity and were recorded to additional paid-in capital at an amount based on the relative fair value of the Tranche Obligations and Class A ordinary shares upon issuance. The obligations to issue shares upon satisfaction of the Tranche 3A and 3B milestones were not considered indexed to the Company’s ordinary shares due to variability of the settlement amount and was required to be recorded as a liability. This liability was estimated to have a fair value of $4.2 million upon issuance in May 2014 and was subsequently remeasured to its fair value of $3.9 million on December 31, 2014, with the decrease in fair value during the year recorded in other expense in the consolidated statement of operations and comprehensive loss. As discussed above, the 2015 Agreement modified the Tranche 3A and 3B milestones. Therefore, the Company remeasured the Tranche 3A and 3B milestone liabilities to fair value in November 2015, which resulted in an increase in fair value of $4.3 million that was recorded in other expense in the consolidated statement of operations and comprehensive loss. All outstanding Tranche Obligations following the modification of the 2015 Agreement were considered indexed to the Company’s ordinary shares and met all additional criteria to be classified as equity and, subsequent to the remeasurement in November 2015, the Tranche Obligations liability was reclassified to additional paid-in capital. At December 31, 2016, the remaining outstanding Tranche Obligations and their respective prespecified milestone criteria were as follows: Tranche Number of Shares Price Per Share Milestone 2 1,846,070 £ 1.00 Commencement of a pivotal trial for CHM 2 773,966 £ 2.07366 Commencement of a pivotal trial for CHM 3 2,725,139 £ 2.07366 First patient treated in a clinical trial for a second program 4 2,725,139 £ 2.07366 First patient treated in a clinical trial for a third program The Company issued 8,070,314 Class A ordinary shares in June 2017 to satisfy the outstanding Tranche Obligations and accordingly, there were no Tranche Obligations outstanding as of December 31, 2017.

Share-Based Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-Based Compensation	9. Share-Based Compensation The Company grants equity awards under its share-based compensation programs, which awards may include share options, RSAs, RSUs and other share-based awards. To date, the share-based awards granted to employees and directors have been in the form of RSAs, RSUs and share options. 2017 Equity Incentive Plan Under the Company’s 2017 Equity Incentive Plan (the “2017 Plan”) approved in September 2017, the Company was authorized to issue a total of 1,500,000 ordinary shares as incentives to the Company’s employees and directors, which includes shares underlying options that may be granted from time to time under the 2017 Plan. In addition, the number of ordinary shares reserved for issuance under the 2017 Plan will automatically increase on January 1 of each year, ending on (and including) January 1, 2027, in an amount equal to 4% of the total number of shares outstanding on December 31 of the preceding calendar year. Pursuant to this provision, 1,341,348 and 1,156,188 ordinary shares were added to the number of available shares pursuant to the 2017 Plan effective January 1, 2019 and 2018. Share Options The Company typically grants employees ten-year share option awards which vest over a four-year service period with 25% of the award vesting on the first anniversary of the vesting commencement date, with the balance vesting ratably on a monthly basis over the remaining three years. As of December 31, 2018, all granted awards contained only service-based vesting conditions. The Company recognizes share-based compensation expense for share option awards based on the grant date fair value of the award on a straight-line basis over the requisite service period. Share Option Valuation The assumptions used in the Black-Scholes option pricing model to determine the fair value of the share options granted to employees and directors during the years ended December 31, 2018 and 2017 were as follows: December 31, December 31, 2018 2017 Expected option life (years) 6 years 6 years Risk-free interest rate 2.63% to 3.01% 2.16% to 2.25% Expected volatility 64.65% to 66.69% 57.17% to 57.43% Expected dividend yield 0.00% 0.00% No share options were granted prior to 2017. Option grant activity for the year ended December 31, 2018 is as follows: Number of Options Weighted Average Exercise Price Weighted Average Remaining Contractual Life Aggregate Intrinsic Value Outstanding at December 31, 2017 191,075 $ 21.00 9.87 years $ 21.00 Granted 965,526 14.47 — — Exercised — — — — Forfeited (56,100 ) 13.60 — — Outstanding at December 31, 2018 1,100,501 $ 15.74 9.17 years $ — Exercisable as of December 31, 2018 51,285 $ 21.56 8.87 years $ — Unvested that are expected to vest and vested as of December 31, 2018 1,100,501 $ 15.74 9.17 years $ — The aggregate intrinsic value of share options is calculated as the difference between the exercise price of the share options and the fair value of the Company’s ordinary shares for those share options that had exercise prices lower than the fair value of the Company’s ordinary shares. In all cases the exercise price was greater than the fair value of the Company’s ordinary shares at year end; therefore, there is no intrinsic value for the share options granted. The weighted average grant-date fair value of share options granted during the years ended December 31, 2018 and 2017 was $8.95 and $13.20, respectively, per share. The total fair value of awards vested for the year ended December 31, 2018 was $0.7 million. At December 31, 2018 and 2017, vested and exercisable shares were 51,285 and none, respectively. As of December 31, 2018, there was $8.3 million Restricted Share Awards and Restricted Share Units Pursuant to the September 2017 reverse stock split, all previously outstanding restricted stock award and restricted share units were converted into to a single class of ordinary shares. The Company typically grants employee incentive shares which vest over a four-year service period with 25% of the award vesting on the first anniversary of the vesting commencement date, with the balance vesting ratably on a quarterly or monthly basis over the remaining three years, unless the awards contain specific performance vesting provisions. As of December 31, 2018 and 2017, all outstanding granted awards contained only service based vesting conditions. The Company recognizes share-based compensation expense for equity awards based on the grant date fair value of the award. For equity awards that vest based on a service condition, the share-based compensation expense is recognized on a straight-line basis over the requisite service period. For Performance Awards, the Company recognizes share-based compensation expense ratably over the requisite service period when the achievement of a performance-based milestone is probable based on the relative satisfaction of the performance condition as of the reporting date. The unvested RSAs are considered legally outstanding shares; however, they are subject to forfeiture in cases where the holders fail to complete the requisite service period with the Company. Unvested RSAs that are forfeited are considered deferred shares and, subject to certain conditions, all deferred shares can be repurchased by the Company for payment of £0.01 to the holder of the deferred shares. The unvested RSUs are not considered outstanding shares until the holders perform the requisite services as defined in the applicable award agreement. In 2018, 10,000 RSUs were issued at the current market price; they remain outstanding as of December 31, 2018. A summary of the changes in the Company’s RSAs and RSUs during the year ended December 31, 2018: Number of Shares Weighted Average Grant Date Fair Value Outstanding at December 31, 2017 1,191,344 4.92 Granted 10,000 14.31 Vested (501,631 ) 4.49 Forfeited (73,781 ) 8.09 Unvested and outstanding at December 31, 2018 625,932 $ 5.00 The company recorded share-based compensation expense as follows (in thousands): December 31, 2018 2017 2016 Research and development $ 1,574 $ 479 $ 152 General and administrative 2,607 894 56 Total share-based compensation $ 4,181 $ 1,373 $ 208 As of December 31, 2018, there was $2.8 million

Earnings per Share

Earnings per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Earnings per Share	10. Earnings per Share Basic earnings per share (EPS) is calculated by dividing the net loss by the weighted average number of ordinary shares outstanding during the period. Unvested RSA’s, although legally issued and outstanding, are not considered outstanding for purposes of calculating basic earnings per share as presented in the following schedule. Diluted EPS is calculated by dividing net income by the weighted average number of shares outstanding plus the dilutive effect, if any, of outstanding stock options, RSAs, and RSUs using the treasury stock method. For all periods presented the Basic and Diluted EPS are the same because the outstanding stock options, RSAs and RSUs were considered anti-dilutive. The following is a presentation of the numerator and denominator used in the calculation of net loss per ordinary share: December 31, (in thousands, except per share data) 2018 2017 2016 Numerator: Net loss, as reported, and available to shareholders - basic and diluted $ (36,862 ) $ (29,686 ) $ (12,198 ) Denominator: Weighted average number of shares used in computing net loss per share - basic and diluted 29,176 18,187 8,954 Basic and Diluted net loss per ordinary share $ (1.26 ) $ (1.63 ) $ (1.36 ) The following common share equivalents have been excluded from the computation of diluted weighted-average shares outstanding, for the periods indicated, as their effect would have been anti-dilutive: December 31, 2018 2017 2016 Unvested restricted share awards and restricted share units 625,932 1,191,344 894,770 Unvested and vested share options 1,100,501 191,075 —

License Agreements

License Agreements	12 Months Ended
	Dec. 31, 2018
License Agreements [Abstract]
License Agreements	11. License Agreements Oxford University Innovation Limited Licenses In November 2013, the Company entered an exclusive license agreement (the “2013 Oxford Agreement”) with Oxford University Innovation (“Oxford”), formerly Isis Innovation Limited, to obtain a license to certain patent rights for the commercial development, manufacture, distribution, use and sale of products and processes resulting from the development of those patent rights. The Company is using the licensed patent rights to develop NSR-REP1 for CHM. As part of the consideration for this license, the Company paid upfront fees of $78,000 and past patent cost of $68,000. The 2013 Oxford Agreement requires the Company to remit fees upon the Company or any sub-licensee meeting certain milestones, as well as up to an aggregate of £375,000 ($507,000) upon the achievement of specified milestones and a low single-digit percentage of net of sales upon the Company achieving regulatory approval, subject to quarterly minimums. In addition, the Company agreed to pay a mid single-digit percentage of all upfront fees, milestone and other one-off payments received by the Company. No amounts were paid or accrued at December 31, 2018 or 2017. In November 2015, the Company entered into five separate exclusive license agreements (the “2015 Oxford Agreements”) to obtain a license to certain patent rights for the commercial development, manufacture, distribution, use and sale of gene therapy products targeting five different types of inherited retinal diseases, including NSR-RPGR and NSR-BEST1, and processes resulting from the development of those patent rights. As part of the consideration for these licenses, the Company paid upfront fees of $575,000 in the aggregate. In addition, the Company agreed to pay Oxford annual maintenance fees for all five licenses until the formal application for regulatory approval of the product is filed, as well as upon the achievement of specified development and commercial milestones and a fee income royalty, which is calculated as a single-digit percentage of all upfront fees, milestone and other one-off payments received by the Company. The Company is also required to pay royalty payments, subject to quarterly minimums, based on a single digit percentage of net sales. In connection with the licenses, Oxford was required to transfer certain manufactured license products as well as all manufacturing documentation. The annual maintenance fee for all five licenses is £60,000 ($80,000) for the year ending December 31, 2018 and will increase to £100,000 ($135,000) through 2025 and beyond. As of December 31, 2018 and 2017, the Company has an accrued obligation of £60,000 ($80,000) and $0, respectively. In October 2017, the Company entered into a separate exclusive license agreement (the “2017 Oxford Agreement”) , including NSR-ABCA4, for the treatment of Stargardt disease. As part of the consideration for this license, the Company paid Oxford an upfront signing fee of £100,000 ($135,000). In addition, the Company agreed to pay Oxford annual maintenance fees and payments upon the achievement of specified development and commercial milestones, sales milestones based on the first achievement of predefined sales thresholds, a royalty on annual net sales, In the event that the Company commits a material breach, has a petition presented for winding up of the business, or passes a resolution for voluntary winding up or challenges the licensed patent rights of Oxford, Oxford can terminate the applicable agreement. The Company has the right to terminate the 2013 Oxford Agreement and 2015 Oxford Agreement if Oxford commits a material breach and at any time after the second anniversary of the applicable agreement. The Company has the right to terminate the 2017 Oxford Agreement if Oxford commits a material breach and at any time after the third anniversary of its execution date upon two months’ written notice. The 2013 Oxford Agreement, 2015 Oxford Agreements and 2017 Oxford Agreement expire on the twentieth anniversary of the applicable effective date or each license will expire upon the Company’s written election when for twelve consecutive months none of these items exist: the licensed patents continue to be in force or subsisting in another country, the Company has market exclusivity throughout the world, the Company has market exclusivity throughout any region or the Company has market exclusivity in any country. The Company can sublicense its rights under the 2013 Oxford Agreement, 2015 Oxford Agreements and 2017 Oxford Agreement. Oxford BioMedica License In December 2013, the Company entered into a non-exclusive license agreement (the “BioMedica License Agreement”) with Oxford BioMedica (UK) Limited (“Oxford BioMedica”) to obtain a license to certain patent rights for the commercial development, manufacture, distribution, use and sale of products and processes resulting from the development of those patent rights. As part of the consideration for this license, the Company paid upfront fees of $100,000. In addition, the Company is also required to pay quarterly royalty payments based on net sales at a low-single digit percentage. The BioMedica License Agreement also requires the Company to make a $100,000 milestone payment upon the grant by the FDA of marketing approval for an AAV product for the treatment of CHM. The Company is allowed to sublicense its rights under the agreement, if this occurs the Company is required to pay an upfront sublicense fee and royalty payments equal to a low-to-mid single-digit amount quarterly as a percentage of sublicense revenue. In the event that the Company commits a material breach, challenges the validity of the licensed patent rights, becomes insolvent or if an order is passed or resolution is passed for the winding up of the Company or if an administrator, administrative receiver or receiver is appointed over the Company’s assets, Oxford BioMedica can terminate the BioMedica License Agreement. The Company can terminate the BioMedica License Agreement at any time and if Oxford BioMedica commits a material breach or becomes insolvent or if an order is passed or resolution is passed for the winding up of Oxford BioMedica or if an administrator, administrative receiver or receiver is appointed over Oxford BioMedica’s assets. The BioMedica License Agreement will expire on the expiration date of the related patent. The Company has not recorded any research and development expense for the years ended December 31, 2018, 2017 and 2016 in connection with the BioMedica License Agreement as the Company has not met any milestones and recognized net sales, which would require the Company to remit payments.

Research and Development Tax Cr

Research and Development Tax Credit	12 Months Ended
	Dec. 31, 2018
Research And Development Tax Credit [Abstract]
Research and Development Tax Credit	12. Research and Development Tax Credit As a company that carries out extensive research and development activities, the Company seeks to benefit from one of two U.K. research and development tax credit cash rebate regimes, the Small and Medium-sized Enterprises R&D Tax Credit Scheme (“SME Scheme”) and the Research and Development Expenditure scheme (“RDEC Scheme”). Under the SME Scheme, the Company’s principal research subsidiary company, NSL, may be eligible to surrender the trading losses that arise from its research and development activities for a payable tax credit of up to approximately 33.4% of eligible research and development expenditures. Qualifying expenditures largely comprise employment costs for research staff, consumables and certain internal overhead costs incurred as part of research projects for which the Company does not receive income. Subcontracted research expenditures are eligible for a cash rebate of up to approximately 21.7%. For certain periods where NSL is not eligible for the SME Scheme, NSL may be eligible for the RDEC Scheme, whereby tax relief was given at 11% of allowable research and development costs, and increased to 12% on January 1, 2018. The RDEC plan is more restrictive with qualifying expenditures mainly comprising of employment costs for research staff. Based on criteria established by HMRC, management of NSL expects a proportion of expenditures being carried out by NSL in relation to its pipeline research, clinical trials management and manufacturing development activities is likely to be eligible for inclusion within one of these two tax credit cash rebate regimes. The Company has recorded U.K. research and development tax credits as an offset to research and development expense in the consolidated statements of operations and comprehensive loss of $7.2 million for the years ended December 31, 2018 and 2016, respectively.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	13. Income Taxes The components of loss before (benefit from) provision for income taxes for the years ended December 31, 2018, 2017 and 2016 are as follows (in thousands): December 31, 2018 2017 2016 U.K. $ (37,765 ) $ (29,978 ) $ (12,194 ) U.S. 463 329 (4 ) Loss before taxes $ (37,302 ) $ (29,649 ) $ (12,198 ) The Company did not record any income tax provision for the year ended December 31, 2016. For the years ended December 31, 2018 and 2017, the Company recorded the following income tax (benefit) provision (in thousands): December 31, 2018 2017 Current expense (benefit): Federal $ (44 ) $ 52 State 12 — Foreign — — Total current expense (benefit) (32 ) 52 Deferred expense (benefit): Federal 192 — State (187 ) (15 ) Foreign (413 ) — Total deferred tax expense (benefit) (408 ) (15 ) Total income tax expense (benefit) $ (440 ) $ 37 A reconciliation of income tax (benefit) expense at the statutory corporate income tax rate to the income tax (benefit) expense at the Company’s effective income tax rate is as follows (in thousands): December 31, 2018 2017 2016 Tax (benefit) at United Kingdom statutory corporate rate $ (7,088 ) $ (5,707 ) $ (2,439 ) Research and development activities 2,728 869 835 Valuation allowance 2,992 4,304 1,122 Change in tax law 1,303 537 437 State taxes, net of federal benefit (164 ) (15 ) — Foreign rate differential 10 49 — Other (221 ) — 45 Income tax expense (benefit) $ (440 ) $ 37 $ - Net deferred tax assets (liabilities) recognized in the Company’s consolidated balance sheet consist of the following (in thousands): December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 9,018 $ 6,549 Intangibles amortization 14 18 Tax credits 404 15 Accruals 24 — Other 18 8 Total deferred tax assets 9,478 6,590 Less: valuation allowance (9,050 ) (6,523 ) Net deferred tax asset (liability) 428 67 Deferred tax liabilities: Depreciation and amortization (1 ) (52 ) Other deferred tax liabilities (427 ) — Total deferred tax liabilities (428 ) (52 ) Net deferred tax asset (liability) $ — $ 15 The Company has evaluated the positive and negative evidence bearing on its ability to realize the deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been established against the net deferred tax assets of December 31, 2018, 2017 and 2016. Management reevaluates the positive and negative evidence at each reporting period. As of December 31, 2018, the Company had $53.0 million of cumulative The Company has not recorded any amounts for unrecognized tax benefits as of December 31, 2018, 2017 and 2016. The Company files income tax returns in the United Kingdom, United States and certain state and local jurisdictions. The income tax returns are generally subject to tax examinations for the tax years ended December 31, 2013 through December 31, 2018. There are currently no pending income tax return examinations.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	14. Commitments and Contingencies License Agreements The Company has entered into several license agreements (see Note 11). In connection with these agreements the Company is required to make a number of milestone payments and annual license maintenance payments. The Company evaluated all milestone payments within the arrangements to estimate the probability of the Company meeting the milestones in accordance with ASC 450, Contingencies that have been achieved and unpaid have been accrued. Legal Proceedings From time to time, the Company may be a party to litigation or subject to claims incident to the ordinary course of business. Although the results of litigation and claims cannot be predicted with certainty, the Company currently believes that the final outcome of these ordinary course matters will not have a material adverse effect on its business. Regardless of the outcome, litigation can have an adverse impact on the Company because of defense and settlement costs, diversion of management resources and other factors. The Company was not a party to any litigation and did not have contingency reserves established for any liabilities as of December 31, 2018 and 2017. Leases On May 24, 2018, NSL entered into an agreement to receive the assignment of the lease for its new office on Midford Place in London, United Kingdom. This office also serves as the corporate headquarters of the Company. The assignment of the lease to NSL became effective on June 8, 2018 and the lease will expire on October 30, 2020. Annual rent is approximately £198,000 ($261,000), payable quarterly. NSL provided the landlord with an upfront security deposit of approximately £119,000 ($157,000), including value added tax. As part of this agreement, NSL also received a one-time rent concession payment from the landlord in the amount of £75,000 ($99,000) plus value added taxes. NSL’s performance under the lease is guaranteed by Nightstar Therapeutics plc. On April 6, 2018, NSI entered into a sublease for its new corporate headquarters in Waltham, Massachusetts. The sublease provides NSI with approximately 12,000 rentable square feet for general office use. The sublease became effective on April 26, 2018 (the “Sublease Commencement Date”) and will expire in March 2021. The initial rent for the office space is approximately $209,000 per annum, increasing every year by approximately 6%. As part of the agreement, NSI arranged for a letter of credit for $58,000 as security for the sublease. On January 10, 2017, the Company entered into a noncancelable sublease for a facility in Lexington, Massachusetts for its U.S. operations. The lease related to the facility commenced on February 1, 2017 and is scheduled to terminate in June 2020. The lease is classified as an operating lease. The Company is committed to making aggregate lease payments of $86,000 in 2018, $89,000 in 2019 and $46,000 in 2020. This Lexington office space is currently considered excess and has been sublet to recover costs. The Company’s future minimum lease obligations as of December 31, 2018 are: Premises Operating Leases (in thousands) 2019 $ 569 2020 427 2021 58 Total obligations $ 1,054 The Company recorded rent expense aggregating $448,000, $205,000 and $138,000 for the years ended December 31, 2018, 2017 and 2016, respectively. Indemnification In the normal course of business, the Company enters into contracts and agreements that contain a variety of representations and warranties and provide for general indemnification. The Company’s exposure under these agreements is unknown because it involves claims that may be made against the Company in the future. To date, the Company has not paid any claims or been required to defend any action related to its indemnification obligations. However, the Company may record charges in the future as a result of these indemnification obligations. In accordance with its Articles of Association, the Company has indemnification obligations to its officers and directors for certain events or occurrences, subject to certain limits, while they are serving at the Company’s request in such capacity. There have been no claims to date, and the Company has director and officer insurance that may enable it to recover a portion of any amounts paid for future potential claims.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party Transactions	15. Related Party Transactions Syncona In the normal course of business, the Company has entered into agreements with controlled or majority-owned entities of Syncona Partner LLP, Syncona Limited and their affiliates, including Freeline Therapeutics (collectively “Syncona”) under which the Company may obtain from or otherwise engage in certain limited research and development activities with these affiliated entities of Syncona in connection with the development of its product candidates. In addition, the Company may provide services to or otherwise engage in certain limited research and development activities with companies affiliated with Syncona, including the provision of clinical manuals and other supportive documents to such affiliated entities. Syncona is a shareholder of the Company and the Chairman of the Board of Directors of the Company serves as the chief investment officer and managing partner of Syncona. The Company has also received accounting and professional services from its principal investor, Syncona from time to time as needed. The Company recorded research and development expenses totaling $53,000, $11,000 and $30,000 for the years ended December 31, 2018, 2017 and 2016, respectively; and recorded $66,000 as other income during the year December 31, 2018. The Company also recorded accounting and professional fees totaling $4,000, $1,200 and $23,000 for the years ended December 31, 2018, 2017 and 2016, respectively. As of December 31, 2018 and 2017, the Company had $28,000 and $13,500, respectively, of accrued and open payments outstanding to affiliates of Syncona. The Company had a lease agreement with The Wellcome Trust Limited, as trustee of The Wellcome Trust, the prior holding company of Syncona. As part of that arrangement, the Company was billed for certain office costs in addition to the leased space. The Company recorded general and administrative expenses totalling $165,000 and $152,000for the years ended December 31, 2017 and 2016, respectively. As of December 31, 2017 and 2016, there was $182,000 and $44,000, respectively, outstanding to The Wellcome Trust. Subsequent to December 31, 2016, The Wellcome Trust is no longer considered an affiliated entity of the Company. The lease with The Wellcome Trust Limited terminated in July 2018. University of Oxford and Related Entities The Company, under various service agreements, receives research and development services from the University of Oxford and its subsidiaries. The University of Oxford is a shareholder of the Company. The Company recorded research and development expenses totaling $566,000, $862,000 and $376,000 for the years ended December 31, 2018, 2017 and 2016, respectively. As of December 31, 2018 and 2017 the Company had $194,000 and $414,000, respectively, included in accrued expenses. See also Note 11 in relation to payments made to Oxford under certain license agreements between Oxford and the Company. MacLaren Agreement In November 2013 and as amended in 2016, the Company entered into a consulting agreement with Oxford University Innovation Ltd. (“Consulting Agreement”), for consulting services of Prof. Robert MacLaren, a former member of the Company’s board of directors. Under the terms of the Consulting Agreement, Prof. MacLaren is to provide the Company with advice and expertise in relation to regulatory submissions, prepare for and attend meetings of the Company’s clinical advisory board, prepare for and attend regulatory meetings, provide scientific and medical advice in relation to the preparation of medical education materials and provide consulting services to the Company’s Chief Medical Officer. Under the terms of the Consulting Agreement, the Company agreed to pay consulting fees for Prof. MacLaren’s services subject to a minimum annual fee of £99,000 ($134,000). In October 2017, the Company’s board of directors awarded Prof. MacLaren 58,000 RSUs which would have vested annually over a four-year period from the date of award. In connection with Prof. MacLaren’s resignation from the Board of Directors (the “Board”), the Board agreed to allow for immediate vesting of 29,000 of the RSUs awarded in October 2017, with the remainder being forfeited as of the same date. The Board also agreed that the Company would pay a bonus to Prof. MacLaren for services provided in 2017 in the amount of $26,000. Under the Consulting Agreement, the Company recorded consulting fees totaling $44,000, $107,000 and $150,000

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
Employee Benefit Plans	16. Employee Benefit Plans In the United Kingdom, the Company makes certain contributions to private benefit pension schemes on behalf of certain of its employees. Effective December 31, 2018, the obligations to the employees’ individual plans was terminated and the Company fulfilled its remaining obligations under those agreements. The Company paid $0.2 million, $0.2 million and $0.2 million in contributions in the years ended December 31, 2018, 2017 and 2016, respectively. In the United States, the Company established a defined contribution savings plan under Section 401(k) of the Internal Revenue Code. This plan covers substantially all U.S. employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. The Company does not currently match employee contributions and accordingly, no matching contributions were recorded for the years ended December 31, 2018, 2017 and 2016.

Selected Quarterly Financial In

Selected Quarterly Financial Information Unaudited	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Information Unaudited	17. Selected Quarterly Financial Information Unaudited The table summarizes the unaudited consolidated financial results of operations for the quarters ended (in thousands, except per share data): March 31 June 30 September 30 December 31 2018 Quarter Ended Research and development $ 6,064 $ 8,052 $ 7,845 $ 9,624 General and administrative 2,776 3,324 3,019 3,453 Other (expense) income, net (5,518 ) 2,946 4,925 4,502 Loss before income taxes (14,358 ) (8,430 ) (5,939 ) (8,575 ) (Benefit from) provision for income taxes — (334 ) 83 (189 ) Net loss $ (14,358 ) $ (8,096 ) $ (6,022 ) $ (8,386 ) Other comprehensive income (loss) $ 6,346 $ (2,681 ) $ (4,329 ) $ (3,788 ) Weighted average ordinary shares outstanding basic and diluted 27,862 28,053 28,139 32,762 Basic and diluted net loss per share $ (0.52 ) $ (0.29 ) $ (0.21 ) $ (0.26 ) 2017 Quarter Ended Research and development $ 2,750 $ 3,542 $ 3,983 $ 10,228 General and administrative 730 677 2,025 3,569 Other income (expense) net 8 (2 ) (2,203 ) 51 Loss before income taxes (3,472 ) (4,221 ) (8,211 ) (13,746 ) (Benefit from) provision for income taxes — — — 37 Net loss $ (3,472 ) $ (4,221 ) $ (8,211 ) $ (13,783 ) Other comprehensive income (loss) $ 143 $ 1,579 $ 1,848 $ 418 Weighted average ordinary shares outstanding basic and diluted 11,680 23,688 21,514 27,336 Basic and diluted net loss per share $ (0.30 ) $ (0.18 ) $ (0.38 ) $ (0.50 )

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	18. Subsequent events On March 4, 2019, the Company entered into an Implementation Agreement with Biogen Switzerland Holdings GmbH and Tungsten Bidco Limited. Pursuant to the terms of the Implementation Agreement, Bidco has agreed to acquire the entire issued and to be issued share capital of the Company for $25.50 in cash per ordinary share, par value £0.01 per share (the “Company Shares”). Under the terms of the Implementation Agreement, the acquisition will be implemented by means of a scheme of arrangement to be undertaken by the Company under Part 26 of the UK Companies Act 2006. The acquisition is subject to, among other things, approval by at least 75 percent in value and a majority in number of the holders of the Company Shares present and voting (in person or by proxy) at the meeting to be convened by order of the High Court of Justice in England and Wales, and the satisfaction or waiver of certain conditions, including law and regulatory authority.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation These consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (U.S. GAAP). The consolidated financial statements include the accounts of the Company and all subsidiaries. All intercompany accounts and transactions have been eliminated. Research and development tax credit receivable was previously included in prepaid and other assets at December 31, 2017 and has been presented as a separate line item in the consolidated financial statements and related footnotes, to conform with the current year presentation.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of income and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual for research and development expenses, the determination of the fair value of share-based awards issued, share-based compensation expense and the recoverability of the Company’s net deferred tax assets and related valuation allowance. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ materially from those estimates.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments that have maturities of three months or less when acquired to be cash equivalents. Cash equivalents consist of investments in money market funds and United States (U.S.) Treasury securities with an original maturity of less than three months.
Restricted Cash	Restricted Cash The Company has entered into a certain lease transaction (refer to Note 14) that requires a security deposit of $58,000 for the duration of the lease contract, which is reported in the Other assets balance line on the consolidated balance sheet. The Company includes the restricted cash balance in cash and cash equivalents when reconciling beginning-of-period and end-of-period total amounts shown on the consolidated statements of cash flows.
Marketable Securities	Marketable securities The Company invests available funds in high-quality Treasury securities that are classified as available-for-sale and carried at fair value. Changes in fair value of available-for-sale securities are recorded in other comprehensive income (loss) as net unrealized gains (losses) on available-for-sale securities. The Company recognized $2.1 million (net of a tax benefit of $0.4 million) in unrealized holding gains for the year ended December 31, 2018. The Company did not recognize any net unrealized gains or losses on marketable securities in any prior period.
Fair Value Measurements	Fair Value Measurements Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: ▪ Level 1—Quoted prices in active markets for identical assets or liabilities. ▪ Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. ▪ Level 3—Unobservable inputs that are supported by little or no market activity that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques For marketable securities, exclusively U.S. Treasury securities, management utilizes values provided by the Company’s investment advisor and compares them to the values published by a third-party source. Management believes that the carrying amounts of the Company’s consolidated financial instruments, including cash equivalents, prepaid expenses, research and development credit receivables, accounts payable and accrued expenses approximate fair value due to the short-term nature of those instruments.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that subject the Company to credit risk consist primarily of cash, cash equivalents and marketable securities. The Company places cash and cash equivalents in established financial institutions. Marketable securities held by the Company consist exclusively of U.S. Treasury securities. The Company has no significant off-balance-sheet risk or concentration of credit risk, such as foreign exchange contracts, options contracts, or other foreign hedging arrangements.
Property and Equipment	Property and Equipment Property and equipment are recorded at cost and depreciated using the straight-line method over the estimated useful lives of the respective assets. As of December 31, 2018 and 2017, the Company’s property and equipment consisted of lab equipment, computer equipment and office equipment, which has an estimated useful life of three years. The Company capitalizes the cost of leasehold improvements and amortizes them over the shorter of the useful life of the asset and the non-cancellable lease term. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the consolidated statement of operations and other comprehensive loss. Expenditures for repairs and maintenance are charged to expense as incurred. The Company evaluates assets for potential impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparing the book values of the assets to the expected future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book values of the assets exceed their fair value. The Company has not recognized any impairment losses from inception through December 31, 2018.
Segment Information	Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker in deciding how to allocate resources and assess performance. The Company and the Company’s chief operating decision maker, the Company’s Chief Executive Officer, view the Company’s operations and manage its business as a single operating segment, which is the business of developing and commercializing gene therapies; however, the Company operates in two geographic regions: United Kingdom and United States.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries, share-based compensation and benefits, depreciation expense, travel, third-party license fees, and external costs of outside vendors engaged to conduct clinical development activities, clinical trials, cost to manufacture clinical trial materials and tax credits associated with research and development activities. Research and development tax credits received from Her Majesty's Revenue & Customs (“HMRC”) are recognized as offsets to research and development expenses.
Research Contract Costs and Accruals	Research Contract Costs and Accruals The Company has entered into various research and development-related contracts with research institutions and other companies. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies or clinical trials, including the phase or completion of events, invoices received and contracted costs. Estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.
Patent Costs	Patent Costs The Company expenses patent application and related legal costs as incurred and classified such costs as general and administrative expenses in the accompanying consolidated statements of operations and comprehensive loss.
Share-Based Compensation	Share-Based Compensation The Company recognizes compensation expense for equity awards based on the grant date fair value of the award on a straight-line basis over the requisite service period. The Company uses the fair value of its ordinary shares to determine the fair value of restricted share awards (“RSAs”) and restricted share units (“RSUs”). The fair value of options is determined using the Black-Scholes option pricing model. The Company accounts for forfeitures as they occur.
Foreign Currency Translation	Foreign Currency Translation The Company maintains its financial statements in the functional currency pounds sterling. Monetary assets and liabilities denominated in currencies other than the functional currency are remeasured into the functional currency at rates of exchange prevailing at the balance sheet dates. Non-monetary assets and liabilities denominated in foreign currencies are remeasured into the functional currency at the exchange rates prevailing at the date of the transaction. Exchange gains or losses arising from foreign currency transactions are included in the determination of net loss for the respective periods. The Company recorded foreign exchange losses of $2.8 million, $2.9 million and $36,000 for the years ended December 31, 2018, 2017 and 2016, respectively. For financial reporting purposes, the financial statements of the Company, which are prepared using the functional currency of pounds sterling, have been translated into the U.S. dollar. Assets and liabilities are translated at the exchange rates at the balance sheet dates, revenue and expenses are translated at the average exchange rates and shareholders’ equity is translated based on historical exchange rates. Translation adjustments are not included in the determination of net loss, but are included as foreign currency adjustments in accumulated other comprehensive income (loss), a component of shareholders’ equity. The Company recorded a loss of $6.7 million, a gain of $4.0 million, and a loss of $1.4 million on foreign currency translation adjustments for the years ended December 31, 2018, 2017 and 2016, respectively.
Income Taxes	Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in its tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the consolidated financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that deferred tax assets will be recovered in the future to the extent management believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed as the amount of benefit to recognize in the consolidated financial statements. The amount of benefits that may be used is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate, as well as the related net interest and penalties. The Company recognizes interest and penalties related to unrecognized tax benefits on the income tax expense line in the accompanying consolidated statement of operations. As of December 31, 2018, 2017 and 2016, no accrued interest or penalties are included on the related tax liability line in the consolidated balance sheet. Research and development tax credits received from HMRC are recognized as offsets to research and development expenses.
Comprehensive Loss	Comprehensive Loss The Company follows the provisions of the Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) Topic 220, Comprehensive Income
Net Loss per Share	Net Loss per Share Basic and diluted net loss per ordinary share is determined by dividing net loss by the weighted average number of ordinary shares outstanding during the period. For all periods presented, the outstanding shares of unvested RSAs and other common share equivalents were excluded from the calculation because the effect would be anti-dilutive. Therefore, the weighted average shares outstanding used to calculate both basic and diluted loss per share are the same.
Smaller Reporting Company Status	Smaller Reporting Company Status In June 2018 the U.S Securities and Exchange Commission amended the thresholds for the smaller reporting company (“SRC”) definition thereby expanding the number of smaller companies eligible to comply with the scaled disclosure requirements. As of December 31, 2018, the Company qualifies for these reduced SRC disclosure requirements. The Company will maintain the SRC status until its revenue and unaffiliated market capitalization exceed the eligibility criteria.
Emerging Growth Company Status	Emerging Growth Company Status As of December 31, 2018, the Company maintained its status as an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act. The Company as an “emerging growth company, may take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not emerging growth companies. The Company may take advantage of these exemptions until the Company is no longer an emerging growth company. The Company may take advantage of these exemptions up until the last day of the fiscal year following the fifth anniversary of its initial public offering that occurred in October 2017, or such earlier time, that it ceases to be an emerging growth company.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently issued accounting pronouncements not yet adopted In February 2016, the FASB issued Auditing Standard Update (“ASU”) 2016-02, Leases. Codification Improvements to Topic 842, Leases Targeted Improvements , In August 2018, the FASB issued ASU 2018-13 ( Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement , reducing certain disclosures concerning the fair value hierarchy. T In ASU 2018-09 (“ASU 2018-09”), Codification Improvements . The ASU 2018-09 provides updates and clarifications to a number of previously issued accountings standards and such updates and clarifications are effective concurrently with the related standards. The Company does not expect the adoption of ASU 2018-09 to In June 2018, the FASB issued ASU 2018-07 , Compensation—Stock Compensation (Topic 718) - Improvements to Nonemployee Share-Based Payment Accounting Revenue from contracts with customers. In June 2016, the FASB issued ASU 2016-13, Financial Instruments Credit Losses (Topic 326) The Company has considered other recent accounting pronouncements and concluded that they are either not applicable to the business, or that the effect is not expected to be material to the consolidated financial statements as a result of future adoption. Recently adopted accounting pronouncements In October 2016, the FASB issued ASU No. 2016-16, Income Taxes (Topic 740) In March 2016, the FASB issued ASU No. 2016-09, Compensation—Stock Compensation (Topic 718 : Improvements to Employee Share-Based Payment Accounting In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash In January 2017, the FASB issued ASU 2017-01, Clarifying the Definition of a Business On August 26, 2016, the FASB issued Accounting Standards Update No 2016-15, Statement of Cash Flows (Topic 230) In January 2016, the FASB issued ASU 2016-01, Financial Instruments – Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities In August 2018, the FASB issued A ccounting Standards Update (“ASU”) , Intangibles – Goodwill and Other – Internal Use Software (Subtopic 350-40), Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract The guidance is effective for the Company in annual periods beginning after December 15, 2019, and interim periods within those annual periods. The Company has the option to apply the guidance prospectively to all implementation costs incurred after the date of adoption or retrospectively. The new guidance requires certain disclosures in the interim and annual period of adoption. On October 1, 2018, the Company adopted ASU 2018-15 on a prospective basis. The adoption did not have a material impact to the consolidated financial statements.

Fair Value of Financial Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets	For cash, current receivables, and accounts payable, the carrying amounts approximate fair value, because of the short maturity of these instruments, and therefore fair value information is not included in the table below (in thousands): December 31, 2018 Total Level 1 Level 2 Level 3 Money market funds and short-term U.S. Treasury securities, included in cash and cash equivalents $ 23,514 $ 23,514 $ — $ — U.S Treasury securities $ 136,385 100,949 35,436 — $ 159,899 $ 124,463 $ 35,436 $ —

Marketable securities (Tables)

Marketable securities (Tables)	12 Months Ended
	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Schedule of Investments in Marketable Securities Classified as Available-for-Sale	As of December 31, 2018, the Company has the following investments in marketable securities classified as available-for-sale (in thousands): Maturity Amortized cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Impact of Foreign Currency Exchange Rate Fluctuations Aggregate Estimated Fair Value U.S. Treasury securities 3 - 9 months $ 133,899 $ — $ (21 ) $ 2,507 $ 136,385

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2018
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following (in thousands): December 31, 2018 2017 Prepayments $ 5,236 $ 2,586 Research grant receivable 576 — Value-added tax receivable 526 905 Interest receivable 128 98 Other 139 96 Total prepaid expenses and other current assets $ 6,605 $ 3,685

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Components of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): December 31, 2018 2017 Lab equipment $ 358 $ 363 Computer and office equipment 554 382 Leasehold improvements 284 — Less: accumulated depreciation (634 ) (390 ) Total property and equipment, net $ 562 $ 355

Accrued Expenses and Other Li_2

Accrued Expenses and Other Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Payables And Accruals [Abstract]
Components of Accrued Expenses and Other Liabilities	Accrued expenses and other liabilities consisted of the following (in thousands): December 31, 2018 2017 Research and development activities $ 6,527 $ 3,779 Compensation and benefits 2,423 1,093 Professional fees 636 410 Other 1,774 907 Total accrued expenses and other liabilities $ 11,360 $ 6,189

Shareholders_ Equity (Tables)

Shareholders’ Equity (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Schedule of Remaining Outstanding Tranche Obligations and Their Respective Prespecified Milestone Criteria	At December 31, 2016, the remaining outstanding Tranche Obligations and their respective prespecified milestone criteria were as follows: Tranche Number of Shares Price Per Share Milestone 2 1,846,070 £ 1.00 Commencement of a pivotal trial for CHM 2 773,966 £ 2.07366 Commencement of a pivotal trial for CHM 3 2,725,139 £ 2.07366 First patient treated in a clinical trial for a second program 4 2,725,139 £ 2.07366 First patient treated in a clinical trial for a third program

Share-Based Compensation (Table

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Assumptions Used in Black-Scholes Option Pricing Model To Determine Fair Value Of Share Options Granted	The assumptions used in the Black-Scholes option pricing model to determine the fair value of the share options granted to employees and directors during the years ended December 31, 2018 and 2017 were as follows: December 31, December 31, 2018 2017 Expected option life (years) 6 years 6 years Risk-free interest rate 2.63% to 3.01% 2.16% to 2.25% Expected volatility 64.65% to 66.69% 57.17% to 57.43% Expected dividend yield 0.00% 0.00%
Summary of Share Option Grant Activity	No share options were granted prior to 2017. Option grant activity for the year ended December 31, 2018 is as follows: Number of Options Weighted Average Exercise Price Weighted Average Remaining Contractual Life Aggregate Intrinsic Value Outstanding at December 31, 2017 191,075 $ 21.00 9.87 years $ 21.00 Granted 965,526 14.47 — — Exercised — — — — Forfeited (56,100 ) 13.60 — — Outstanding at December 31, 2018 1,100,501 $ 15.74 9.17 years $ — Exercisable as of December 31, 2018 51,285 $ 21.56 8.87 years $ — Unvested that are expected to vest and vested as of December 31, 2018 1,100,501 $ 15.74 9.17 years $ —
Summary of Changes in the Company's RSAs and RSUs	A summary of the changes in the Company’s RSAs and RSUs during the year ended December 31, 2018: Number of Shares Weighted Average Grant Date Fair Value Outstanding at December 31, 2017 1,191,344 4.92 Granted 10,000 14.31 Vested (501,631 ) 4.49 Forfeited (73,781 ) 8.09 Unvested and outstanding at December 31, 2018 625,932 $ 5.00
Summary of Recorded Share-based Compensation Expense	The company recorded share-based compensation expense as follows (in thousands): December 31, 2018 2017 2016 Research and development $ 1,574 $ 479 $ 152 General and administrative 2,607 894 56 Total share-based compensation $ 4,181 $ 1,373 $ 208

Earnings per Share (Tables)

Earnings per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Presentation of Numerator and Denominator Used in Calculation of Net Loss per Ordinary Share	The following is a presentation of the numerator and denominator used in the calculation of net loss per ordinary share: December 31, (in thousands, except per share data) 2018 2017 2016 Numerator: Net loss, as reported, and available to shareholders - basic and diluted $ (36,862 ) $ (29,686 ) $ (12,198 ) Denominator: Weighted average number of shares used in computing net loss per share - basic and diluted 29,176 18,187 8,954 Basic and Diluted net loss per ordinary share $ (1.26 ) $ (1.63 ) $ (1.36 )
Common Share Equivalents Excluded from the Computation of Diluted Weighted-average Shares Outstanding	The following common share equivalents have been excluded from the computation of diluted weighted-average shares outstanding, for the periods indicated, as their effect would have been anti-dilutive: December 31, 2018 2017 2016 Unvested restricted share awards and restricted share units 625,932 1,191,344 894,770 Unvested and vested share options 1,100,501 191,075 —

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Components of Loss Before (Benefit from) Provision for Income Taxes	The components of loss before (benefit from) provision for income taxes for the years ended December 31, 2018, 2017 and 2016 are as follows (in thousands): December 31, 2018 2017 2016 U.K. $ (37,765 ) $ (29,978 ) $ (12,194 ) U.S. 463 329 (4 ) Loss before taxes $ (37,302 ) $ (29,649 ) $ (12,198 )
Schedule of Income Tax (Benefit) Provision	The Company did not record any income tax provision for the year ended December 31, 2016. For the years ended December 31, 2018 and 2017, the Company recorded the following income tax (benefit) provision (in thousands): December 31, 2018 2017 Current expense (benefit): Federal $ (44 ) $ 52 State 12 — Foreign — — Total current expense (benefit) (32 ) 52 Deferred expense (benefit): Federal 192 — State (187 ) (15 ) Foreign (413 ) — Total deferred tax expense (benefit) (408 ) (15 ) Total income tax expense (benefit) $ (440 ) $ 37
Schedule of Reconciliation of Income Tax (Benefit) Expense	A reconciliation of income tax (benefit) expense at the statutory corporate income tax rate to the income tax (benefit) expense at the Company’s effective income tax rate is as follows (in thousands): December 31, 2018 2017 2016 Tax (benefit) at United Kingdom statutory corporate rate $ (7,088 ) $ (5,707 ) $ (2,439 ) Research and development activities 2,728 869 835 Valuation allowance 2,992 4,304 1,122 Change in tax law 1,303 537 437 State taxes, net of federal benefit (164 ) (15 ) — Foreign rate differential 10 49 — Other (221 ) — 45 Income tax expense (benefit) $ (440 ) $ 37 $ -
Schedule of Net Deferred Tax Assets (Liabilities)	Net deferred tax assets (liabilities) recognized in the Company’s consolidated balance sheet consist of the following (in thousands): December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 9,018 $ 6,549 Intangibles amortization 14 18 Tax credits 404 15 Accruals 24 — Other 18 8 Total deferred tax assets 9,478 6,590 Less: valuation allowance (9,050 ) (6,523 ) Net deferred tax asset (liability) 428 67 Deferred tax liabilities: Depreciation and amortization (1 ) (52 ) Other deferred tax liabilities (427 ) — Total deferred tax liabilities (428 ) (52 ) Net deferred tax asset (liability) $ — $ 15

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Obligations	The Company’s future minimum lease obligations as of December 31, 2018 are: Premises Operating Leases (in thousands) 2019 $ 569 2020 427 2021 58 Total obligations $ 1,054

Selected Quarterly Financial _2

Selected Quarterly Financial Information Unaudited (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Summary of Unaudited Consolidated Financial Results of Operations	The table summarizes the unaudited consolidated financial results of operations for the quarters ended (in thousands, except per share data): March 31 June 30 September 30 December 31 2018 Quarter Ended Research and development $ 6,064 $ 8,052 $ 7,845 $ 9,624 General and administrative 2,776 3,324 3,019 3,453 Other (expense) income, net (5,518 ) 2,946 4,925 4,502 Loss before income taxes (14,358 ) (8,430 ) (5,939 ) (8,575 ) (Benefit from) provision for income taxes — (334 ) 83 (189 ) Net loss $ (14,358 ) $ (8,096 ) $ (6,022 ) $ (8,386 ) Other comprehensive income (loss) $ 6,346 $ (2,681 ) $ (4,329 ) $ (3,788 ) Weighted average ordinary shares outstanding basic and diluted 27,862 28,053 28,139 32,762 Basic and diluted net loss per share $ (0.52 ) $ (0.29 ) $ (0.21 ) $ (0.26 ) 2017 Quarter Ended Research and development $ 2,750 $ 3,542 $ 3,983 $ 10,228 General and administrative 730 677 2,025 3,569 Other income (expense) net 8 (2 ) (2,203 ) 51 Loss before income taxes (3,472 ) (4,221 ) (8,211 ) (13,746 ) (Benefit from) provision for income taxes — — — 37 Net loss $ (3,472 ) $ (4,221 ) $ (8,211 ) $ (13,783 ) Other comprehensive income (loss) $ 143 $ 1,579 $ 1,848 $ 418 Weighted average ordinary shares outstanding basic and diluted 11,680 23,688 21,514 27,336 Basic and diluted net loss per share $ (0.30 ) $ (0.18 ) $ (0.38 ) $ (0.50 )

Nature of the Business - Additi

Nature of the Business - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	Oct. 02, 2018	Oct. 02, 2017	Dec. 31, 2018
Minimum
Nature Of Business [Line Items]
Number of months for operating plan sufficiently funded by cash resources			12 months
American Depositary Shares
Nature Of Business [Line Items]
Net proceeds after deducting underwriting discounts and commissions and estimated offering expenses	$ 77.2	$ 77.4
American Depositary Shares | IPO
Nature Of Business [Line Items]
Issuance of ordinary shares, shares	4,600,000	6,164,000
Price per share		$ 14

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended			68 Months Ended
	Dec. 31, 2018USD ($)Segment	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2018USD ($)	Apr. 06, 2018USD ($)
Summary Of Significant Accounting Policies [Line Items]
Net unrealized gains (losses) on available-for-sale securities recorded in other comprehensive income (loss) before tax	$ 2,100,000	$ 0
Net unrealized gains (losses) on available-for-sale securities recorded in other comprehensive income (loss) tax	$ (400,000)
Property and equipment, impairment losses recognized				$ 0
Number of operating segments | Segment	2
Foreign currency exchange loss	$ 2,800,000	2,900,000	$ 36,000
Gain (loss) related to foreign currency translation	(6,745,000)	3,988,000	(1,385,000)
Accrued interest	0	0	0	0
Accrued penalties	$ 0	$ 0	$ 0	$ 0
Minimum
Summary Of Significant Accounting Policies [Line Items]
Percentage of income tax realized upon settlement	50.00%
Lab Equipment
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, useful life	3 years	3 years
Computer Equipment
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, useful life	3 years	3 years
Office Equipment
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, useful life	3 years	3 years
Leasehold Improvements
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, amortization description	the shorter of the useful life of the asset and the non-cancellable lease term
Other Assets
Summary Of Significant Accounting Policies [Line Items]
Security deposits for duration of lease contracts					$ 58,000

Fair Value of Financial Asset_3

Fair Value of Financial Assets and Liabilities - Fair Value of Financial Assets (Details) $ in Thousands	Dec. 31, 2018USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value	$ 159,899
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value	124,463
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value	35,436
Money Market Funds and Short-term U.S. Treasury Securities, Included in Cash and Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value	23,514
Money Market Funds and Short-term U.S. Treasury Securities, Included in Cash and Cash Equivalents | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value	23,514
U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value	136,385
U.S. Treasury Securities | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value	100,949
U.S. Treasury Securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value	$ 35,436

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities - Additional Information (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value	$ 159,899,000
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value		$ 0
Liabilities measured at fair value	$ 0	$ 0

Marketable securities - Schedul

Marketable securities - Schedule of Investments in Marketable Securities Classified as Available-for-Sale (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, aggregate estimated fair value	$ 159,900
U.S. Treasury Securities
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, amortized cost	133,899
Available-for-sale securities, gross unrealized holding losses	(21)
Available-for-sale securities, impact of foreign currency exchange rate fluctuations	2,507
Available-for-sale securities, aggregate estimated fair value	$ 136,385
U.S. Treasury Securities | Minimum
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, maturity	3 months
U.S. Treasury Securities | Maximum
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, maturity	9 months

Marketable securities - Additio

Marketable securities - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities aggregated	$ 159,900,000
Marketable securities	136,385,000	$ 0
Other comprehensive income net gain related to changes in securities fair value	2,081,000
Net unrealized gains (losses) on available-for-sale securities recorded in other comprehensive income (loss) tax	(400,000)
Unrealized loss position for more than one year	0
Foreign currency gains on settlement of maturing marketable securities	6,741,000
Foreign Currency Gain (Loss)
Schedule Of Available For Sale Securities [Line Items]
Foreign currency gains on settlement of maturing marketable securities	6,700,000
Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities aggregated	23,500,000
U.S. Treasury Securities
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities aggregated	$ 136,385,000

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepayments	$ 5,236	$ 2,586
Research grant receivable	576
Value-added tax receivable	526	905
Interest receivable	128	98
Other	139	96
Total prepaid expenses and other current assets	$ 6,605	$ 3,685

Property and Equipment, Net - C

Property and Equipment, Net - Components of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Less: accumulated depreciation	$ (634)	$ (390)
Total property and equipment, net	562	355
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	358	363
Computer and Office Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	554	$ 382
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 284

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Depreciation expense	$ 276,000	$ 206,000	$ 138,000

Accrued Expenses and Other Li_3

Accrued Expenses and Other Liabilities - Components of Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Research and development activities	$ 6,527	$ 3,779
Compensation and benefits	2,423	1,093
Professional fees	636	410
Other	1,774	907
Total accrued expenses and other liabilities	$ 11,360	$ 6,189

Shareholders' Equity - Addition

Shareholders' Equity - Additional Information (Details)	Oct. 02, 2018USD ($)$ / sharesshares	Oct. 02, 2017USD ($)$ / sharesshares	Jun. 27, 2017USD ($)shares	Dec. 31, 2014USD ($)	Jun. 30, 2017USD ($)$ / sharesshares	Nov. 30, 2015USD ($)	May 31, 2014USD ($)	Dec. 31, 2018USD ($)Voteshares	Dec. 31, 2017USD ($)shares	Dec. 31, 2016USD ($)	Mar. 31, 2017£ / sharesshares
Class Of Stock [Line Items]
Proceeds of issuance of ordinary shares, net of issuance costs								$ 77,164,000	$ 141,093,000	$ 11,317,000
Number of votes per ordinary shares | Vote								1
Dividends								$ 0
Issuance of shares | shares								33,485,866	28,904,714
Estimated fair value of tranche obligation liability							$ 4,200,000
Change in fair value of tranche obligation liability				$ 3,900,000		$ (4,300,000)
Ordinary shares, outstanding | shares								33,485,866	28,904,714
Tranche Obligations
Class Of Stock [Line Items]
Ordinary shares, outstanding | shares									0
March 2017 Agreement
Class Of Stock [Line Items]
Termination date								2017-06
American Depositary Shares
Class Of Stock [Line Items]
Net proceeds after deducting underwriting discounts and commissions and estimated offering expenses	$ 77,200,000	$ 77,400,000
American Depositary Shares | Nightstar Therapeutics Limited
Class Of Stock [Line Items]
Net proceeds after deducting underwriting discounts and commissions and estimated offering expenses		$ 2,900,000
American Depositary Shares | IPO
Class Of Stock [Line Items]
Issuance of ordinary shares, shares | shares	4,600,000	6,164,000
Price per share | $ / shares		$ 14
American Depositary Shares | Underwritten Public Offering
Class Of Stock [Line Items]
Issuance of ordinary shares, shares | shares	4,600,000
Price per share | $ / shares	$ 18
Proceeds of issuance of ordinary shares, net of issuance costs	$ 77,200,000
Underwriting discounts and commissions and offering expenses	$ 700,000
Class A Ordinary Shares
Class Of Stock [Line Items]
Issuance of shares | shares					8,070,314
Class A Ordinary Shares | March 2017 Agreement
Class Of Stock [Line Items]
Proceeds of issuance of ordinary shares, net of issuance costs			$ 18,800,000
Subscription of aggregate shares | shares											8,070,314
Issuance of shares | shares			8,070,314
Equity issuance costs			$ 50,000
Class A Ordinary Shares | March 2017 Agreement | Common Stock Shares Subscriptions Price One
Class Of Stock [Line Items]
Subscription of aggregate shares | shares											1,846,071
Shares issued, price per share | £ / shares											£ 1
Class A Ordinary Shares | March 2017 Agreement | Common Stock Shares Subscriptions Price Two
Class Of Stock [Line Items]
Subscription of aggregate shares | shares											6,224,243
Shares issued, price per share | £ / shares											£ 2.0737
Class A Ordinary Shares | June 2017 Agreement
Class Of Stock [Line Items]
Issuance of ordinary shares, shares | shares					11,203,837
Price per share | $ / shares					$ 4.01648
Proceeds of issuance of ordinary shares, net of issuance costs					$ 45,000,000
Stock issuance costs					$ 295,000

Shareholders' Equity - Summary

Shareholders' Equity - Summary of Remaining Outstanding Tranche Obligations and Respective Prespecified Milestone Criteria (Details) - £ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Number of Shares		33,485,866	28,904,714
Remaining Outstanding Obligations Tranche 2
Class Of Stock [Line Items]
Number of Shares	1,846,070
Price Per Share	£ 1
Milestone	Commencement of a pivotal trial for CHM
Remaining Outstanding Obligations Tranche 2
Class Of Stock [Line Items]
Number of Shares	773,966
Price Per Share	£ 2.07366
Milestone	Commencement of a pivotal trial for CHM
Remaining Outstanding Obligations Tranche 3
Class Of Stock [Line Items]
Number of Shares	2,725,139
Price Per Share	£ 2.07366
Milestone	First patient treated in a clinical trial for a second program
Remaining Outstanding Obligations Tranche 4
Class Of Stock [Line Items]
Number of Shares	2,725,139
Price Per Share	£ 2.07366
Milestone	First patient treated in a clinical trial for a third program

Share-Based Compensation - Addi

Share-Based Compensation - Additional Information (Details)	Jan. 01, 2019shares	Jan. 01, 2018shares	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017$ / sharesshares	Dec. 31, 2016shares	Dec. 31, 2018£ / shares	Sep. 30, 2017shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Repurchase of deferred shares, price per share | £ / shares						£ 0.01
Share Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation, award expiration period			10 years
Share-based compensation, award vesting period			4 years
Share-based compensation arrangement award vesting rights description			the balance vesting ratably on a monthly basis over the remaining three years
Share-based compensation award remaining vesting period			3 years
Share-based compensation arrangement by share-based payment award, options granted			965,526		0
Share-based compensation arrangement by share-based payment award, intrinsic value of options granted | $			$ 0
Weighted average grant-date fair value of share options granted | $ / shares			$ 8.95	$ 13.20
Fair value of awards vested | $			$ 700,000
Share-based compensation arrangement by share-based payment award, Vested and exercisable shares			51,285	0
Unrecognized share-based compensation costs related to unvested outstanding awards | $			$ 8,300,000
Weighted-average periods of cost expected to be recognized			3 years
Share Options | Share-Based Compensation Award, Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation percentage of award vesting rights			25.00%
Unvested Restricted Shares and Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation, award vesting period			4 years
Share-based compensation arrangement award vesting rights description			the balance vesting ratably on a quarterly or monthly basis over the remaining three years
Share-based compensation award remaining vesting period			3 years
Unrecognized share-based compensation costs related to unvested outstanding awards | $			$ 2,800,000
Weighted-average periods of cost expected to be recognized			2 years 4 months 24 days
Number of shares issued			10,000
Number of shares oustanding			625,932	1,191,344
Intrinsic value of unvested awards outstanding | $			$ 7,200,000
Fair value of awards vested | $			$ 7,900,000
Unvested Restricted Shares and Restricted Stock Units | Share-Based Compensation Award, Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation percentage of award vesting rights			25.00%
Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares issued			10,000
Number of shares oustanding			10,000
2017 Equity Incentive Plan | Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of ordinary shares authorized under share-based compensation plan							1,500,000
Percentage threshold of outstanding shares increased annually under share-based compensation plan			4.00%
Description of share-based compensation plan			the number of ordinary shares reserved for issuance under the 2017 Plan will automatically increase on January 1 of each year, ending on (and including) January 1, 2027, in an amount equal to 4% of the total number of shares outstanding on December 31 of the preceding calendar year
Number of ordinary shares added under share-based compensation plan		1,156,188
2017 Equity Incentive Plan | Employees and Directors | Subsequent Event
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of ordinary shares added under share-based compensation plan	1,341,348

Share-Based Compensation - Assu

Share-Based Compensation - Assumptions Used in Black-Scholes Option Pricing Model To Determine Fair Value Of Share Options Granted (Details) - Employees and Directors	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected option life (years)	6 years	6 years
Risk-free interest rate, minimum	2.63%	2.16%
Risk-free interest rate, maximum	3.01%	2.25%
Expected volatility, minimum	64.65%	57.17%
Expected volatility, maximum	66.69%	57.43%
Expected dividend yield	0.00%	0.00%

Share-Based Compensation - Summ

Share-Based Compensation - Summary of Share Option Grant Activity (Details) - Share Options - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2016
Number of Options
Number of Options, Outstanding beginning balance	191,075
Number of Options, Granted	965,526	0
Number of Options, Forfeited	(56,100)
Number of Options, Outstanding ending balance	1,100,501
Number of Options, Exercisable as of December 31, 2018	51,285
Number of Options, Unvested that are expected to vest and vested as of December 31, 2018	1,100,501
Weighted Average Exercise Price
Weighted Average Exercise Price, Outstanding beginning balance	$ 21
Weighted Average Exercise Price, Granted	14.47
Weighted Average Exercise Price, Forfeited	13.60
Weighted Average Exercise Price, Outstanding ending balance	15.74
Weighted Average Exercise Price, Exercisable as of December 31, 2018	21.56
Weighted Average Exercise Price, Unvested that are expected to vest and vested as of December 31, 2018	$ 15.74
Weighted Average Remaining Contractual Life (in years)
Weighted Average Remaining Contractual Life, Outstanding	9 years 2 months 1 day
Weighted Average Remaining Contractual Life (in years), Exercisable as of December 31, 2018	8 years 10 months 13 days
Weighted Average Remaining Contractual Life (in years), Unvested that are expected to vest and vested as of December 31, 2018	9 years 2 months 1 day
Aggregate Intrinsic Value
Aggregate Intrinsic Value, Outstanding at December 31, 2017	$ 21

Share-Based Compensation - Su_2

Share-Based Compensation - Summary of Changes in the Company's RSAs and RSUs (Details) - Unvested Restricted Shares and Restricted Stock Units	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Number of Restricted Shares
Beginning balance | shares	1,191,344
Granted | shares	10,000
Vested | shares	(501,631)
Forfeited | shares	(73,781)
Ending balance | shares	625,932
Weighted Average Grant Date Fair Value
Beginning balance | $ / shares	$ 4.92
Granted | $ / shares	14.31
Vested | $ / shares	4.49
Forfeited | $ / shares	8.09
Ending balance | $ / shares	$ 5

Share-Based Compensation - Su_3

Share-Based Compensation - Summary of Recorded Share-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total share-based compensation	$ 4,181	$ 1,373	$ 208
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total share-based compensation	1,574	479	152
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total share-based compensation	$ 2,607	$ 894	$ 56

Earnings per Share - Presentati

Earnings per Share - Presentation of Numerator and Denominator Used in Calculation of Net Loss per Ordinary Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss, as reported, and available to shareholders - basic and diluted	$ (8,386)	$ (6,022)	$ (8,096)	$ (14,358)	$ (13,783)	$ (8,211)	$ (4,221)	$ (3,472)	$ (36,862)	$ (29,686)	$ (12,198)
Denominator:
Weighted average number of shares used in computing net loss per share - basic and diluted	32,762	28,139	28,053	27,862	27,336	21,514	23,688	11,680	29,176,000	18,187,000	8,954,000
Basic and Diluted net loss per ordinary share	$ (0.26)	$ (0.21)	$ (0.29)	$ (0.52)	$ (0.50)	$ (0.38)	$ (0.18)	$ (0.30)	$ (1.26)	$ (1.63)	$ (1.36)

Earnings per Share - Common Sha

Earnings per Share - Common Share Equivalents Excluded from the Computation of Diluted Weighted-average Shares Outstanding (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Unvested Restricted Share Awards and Restricted Share Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Unvested restricted share awards and restricted share units	625,932	1,191,344	894,770
Unvested and Vested Share Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Unvested restricted share awards and restricted share units	1,100,501	191,075

License Agreements - Additional

License Agreements - Additional Information (Details)	1 Months Ended						3 Months Ended								12 Months Ended
	Oct. 31, 2017USD ($)	Oct. 31, 2017GBP (£)	Nov. 30, 2015USD ($)AgreementDisease	Dec. 31, 2013USD ($)	Nov. 30, 2013USD ($)	Nov. 30, 2013GBP (£)	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2018GBP (£)	Dec. 31, 2017USD ($)	Dec. 31, 2017GBP (£)	Dec. 31, 2016USD ($)
License Agreements [Line Items]
Research and development							$ 9,624,000	$ 7,845,000	$ 8,052,000	$ 6,064,000	$ 10,228,000	$ 3,983,000	$ 3,542,000	$ 2,750,000	$ 31,585,000		$ 20,502,000		$ 10,165,000
2013 Oxford Agreement
License Agreements [Line Items]
License agreement entered date															2013-11	2013-11
Upfront fees					$ 78,000
Past patent costs					68,000
License agreement paid or accrued amount															$ 0		0
2013 Oxford Agreement | Maximum
License Agreements [Line Items]
Milestone fees payments upon achievement of specified sales					$ 507,000	£ 375,000
2015 Oxford Agreements
License Agreements [Line Items]
License agreement entered date															2015-11	2015-11
Upfront fees			$ 575,000
Number of separate exclusive license agreements | Agreement			5
Number of different types of inherited retinal diseases | Disease			5
Annual licenses maintenance fee															$ 80,000	£ 60,000
Increase of annual licenses maintenance fee, through 2025 and beyond															135,000,000	100,000,000
License accrued obligation															$ 80,000	£ 60,000	0
2017 Oxford Agreement
License Agreements [Line Items]
License agreement entered date															2017-10	2017-10
Upfront fees	$ 135,000	£ 100,000
Annual licenses maintenance fee															$ 13,000	£ 10,000		£ 0
License accrued obligation															$ 13,000	10,000	0
2017 Oxford Agreement | Maximum
License Agreements [Line Items]
Annual licenses maintenance fee, through 2025 and beyond | £																£ 20,000
Oxford BioMedica License
License Agreements [Line Items]
License agreement entered date															2013-12	2013-12
Upfront fees				$ 100,000
Milestone payment upon approval of a product				$ 100,000
Research and development															$ 0		$ 0		$ 0

Research and Development Tax _2

Research and Development Tax Credit - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Research And Development Tax Credit [Line Items]
Tax relief percentage of allowable research and development costs	11.00%			12.00%
U.K.
Research And Development Tax Credit [Line Items]
Maximum parentage of tax credit payable for eligible research and development expenditures	33.40%
Maximum percentage of cash rebate eligible by subcontracted research expenditures	21.70%
Research and development foreign tax credit used offset research and development expense	$ 7.2	$ 1.3	$ 2.5

Income Taxes - Schedule of Comp

Income Taxes - Schedule of Components of Loss Before (Benefit from) Provision for Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Line Items]
Loss before taxes	$ (8,575)	$ (5,939)	$ (8,430)	$ (14,358)	$ (13,746)	$ (8,211)	$ (4,221)	$ (3,472)	$ (37,302)	$ (29,649)	$ (12,198)
U.K.
Income Tax Disclosure [Line Items]
Loss before taxes									(37,765)	(29,978)	(12,194)
U.S.
Income Tax Disclosure [Line Items]
Loss before taxes									$ 463	$ 329	$ (4)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details)	3 Months Ended				12 Months Ended
	Dec. 31, 2018USD ($)TaxReturnExamination	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2018USD ($)TaxReturnExamination	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Income Tax Disclosure [Line Items]
Income tax provision	$ (189,000)	$ 83,000	$ (334,000)	$ 37,000	$ (440,000)	$ 37,000	$ 0
Unrecognized tax benefits	$ 0			$ 0	$ 0	$ 0	$ 0
Income tax examination, description					The income tax returns are generally subject to tax examinations for the tax years ended December 31, 2013 through December 31, 2018.
Number of pending income tax return examinations | TaxReturnExamination	0				0
U.K.
Income Tax Disclosure [Line Items]
Cumulative gross U.K. net operating loss carryforwards	$ 53,000,000				$ 53,000,000
Federal
Income Tax Disclosure [Line Items]
Research and development tax credit carryforwards	200,000				200,000
State
Income Tax Disclosure [Line Items]
Research and development tax credit carryforwards	$ 200,000				$ 200,000

Income Taxes - Schedule of Inco

Income Taxes - Schedule of Income Tax (Benefit) Provision (Details) - USD ($)	3 Months Ended				12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current expense (benefit):
Federal					$ (44,000)	$ 52,000
State					12,000
Total current expense (benefit)					(32,000)	52,000
Deferred expense (benefit):
Federal					192,000
State					(187,000)	(15,000)
Foreign					(413,000)
Total deferred tax expense (benefit)					(408,000)	(15,000)
Total income tax expense (benefit)	$ (189,000)	$ 83,000	$ (334,000)	$ 37,000	$ (440,000)	$ 37,000	$ 0

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Income Tax (Benefit) Expense (Details) - USD ($)	3 Months Ended				12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Tax (benefit) at United Kingdom statutory corporate rate					$ (7,088,000)	$ (5,707,000)	$ (2,439,000)
Research and development activities					2,728,000	869,000	835,000
Valuation allowance					2,992,000	4,304,000	1,122,000
Change in tax law					1,303,000	537,000	437,000
State taxes, net of federal benefit					(164,000)	(15,000)
Foreign rate differential					10,000	49,000
Other					(221,000)		45,000
Total income tax expense (benefit)	$ (189,000)	$ 83,000	$ (334,000)	$ 37,000	$ (440,000)	$ 37,000	$ 0

Income Taxes - Schedule of Net

Income Taxes - Schedule of Net Deferred Tax Assets (Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 9,018	$ 6,549
Intangibles amortization	14	18
Tax credits	404	15
Accruals	24
Other	18	8
Total deferred tax assets	9,478	6,590
Less: valuation allowance	(9,050)	(6,523)
Net deferred tax asset (liability)	428	67
Deferred tax liabilities:
Depreciation and amortization	(1)	(52)
Other deferred tax liabilities	(427)
Total deferred tax liabilities	$ (428)	(52)
Net deferred tax asset (liability)		$ 15

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	May 24, 2018USD ($)	May 24, 2018GBP (£)	Apr. 06, 2018USD ($)ft²	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	May 24, 2018GBP (£)
Commitments And Contingencies [Line Items]
Litigation contingency reserves liability				$ 0	$ 0
Operating lease, aggregate lease payments, 2019				569,000
Operating lease, aggregate lease payments, 2020				$ 427,000
Operating lease payments commencement date				Feb. 1, 2017
Operating lease terminate period				2020-06
Rent expense associated with lease				$ 448,000	$ 205,000	$ 138,000
Lexington, Massachusetts, U.S.
Commitments And Contingencies [Line Items]
Operating lease, aggregate lease payments, 2018				86,000
Operating lease, aggregate lease payments, 2019				89,000
Operating lease, aggregate lease payments, 2020				$ 46,000
NightstaRx Limited
Commitments And Contingencies [Line Items]
Lease agreement effective date	Jun. 8, 2018	Jun. 8, 2018
Sublease expiration date	Oct. 30, 2020	Oct. 30, 2020
Annual rent	$ 261,000	£ 198,000
Upfront security deposit	157,000						£ 119,000
One-time concession payment from landlord	$ 99,000						£ 75,000
Nightstar, Inc. (“NSI”)
Commitments And Contingencies [Line Items]
Sublease expiration date			Mar. 31, 2021
Annual rent			$ 209,000
Upfront security deposit			$ 58,000
Rentable square feet | ft²			12,000
Sublease commencement date			Apr. 26, 2018
Operating lease of annual increase in percentage			6.00%

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Lease Obligations (Details) $ in Thousands	Dec. 31, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 569
2020	427
2021	58
Total obligations	$ 1,054

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details)	1 Months Ended	3 Months Ended								12 Months Ended
	Oct. 31, 2017shares	Dec. 31, 2018USD ($)shares	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)shares	Dec. 31, 2018GBP (£)shares	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Related Party Transaction [Line Items]
Research and development expenses		$ 9,624,000	$ 7,845,000	$ 8,052,000	$ 6,064,000	$ 10,228,000	$ 3,983,000	$ 3,542,000	$ 2,750,000	$ 31,585,000		$ 20,502,000	$ 10,165,000
General and administrative		3,453,000	$ 3,019,000	$ 3,324,000	$ 2,776,000	3,569,000	$ 2,025,000	$ 677,000	$ 730,000	12,572,000		7,001,000	2,055,000
Consulting fees payment obligations outstanding		636,000				410,000				$ 636,000		410,000
RSUs
Related Party Transaction [Line Items]
Granted | shares										10,000	10,000
MacLaren Agreement
Related Party Transaction [Line Items]
Consulting fees										$ 182,000
Syncona
Related Party Transaction [Line Items]
Research and development expenses										53,000		11,000	30,000
Other income										66,000
Consulting fees										4,000		1,200	23,000
Accrued and open payments outstanding to affiliates		28,000				13,500				$ 28,000		13,500
The Wellcome Trust
Related Party Transaction [Line Items]
General and administrative												165,000	152,000
Outstanding payables						182,000						182,000	44,000
Lease termination period										2018-07	2018-07
University of Oxford and Related Entities
Related Party Transaction [Line Items]
Research and development expenses										$ 566,000		862,000	376,000
University of Oxford and Related Entities | Accrued Expenses
Related Party Transaction [Line Items]
Research and development expenses										194,000		414,000
Oxford University Innovation Ltd | MacLaren Agreement
Related Party Transaction [Line Items]
Consulting fees										$ 44,000		107,000	$ 150,000
License agreement entered date										2013-11	2013-11
Bonus for services provided		26,000								$ 26,000
License agreement terminates date										2018-08	2018-08
Consulting fees payment obligations outstanding		$ 124,000				$ 8,000				$ 124,000		$ 8,000
Oxford University Innovation Ltd | MacLaren Agreement | RSUs
Related Party Transaction [Line Items]
Granted | shares	58,000
Share-based compensation, award vesting period										4 years	4 years
Restricted shares options available for immediately vesting | shares		29,000								29,000
Oxford University Innovation Ltd | MacLaren Agreement | Minimum
Related Party Transaction [Line Items]
Consulting fees										$ 134,000	£ 99,000

Employee Benefit Plans - Additi

Employee Benefit Plans - Additional Information (Details) - Defined Benefit Pension Plan - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
U.K.
Defined Contribution Plan Disclosure [Line Items]
Defined benefit contribution amount	$ 200,000	$ 200,000	$ 200,000
UNITED STATES
Defined Contribution Plan Disclosure [Line Items]
Matching contributions by employer recorded amount	$ 0	$ 0	$ 0

Selected Quarterly Financial _3

Selected Quarterly Financial Information Unaudited - Summary of Unaudited Consolidated Financial Results of Operations (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Research and development	$ 9,624,000	$ 7,845,000	$ 8,052,000	$ 6,064,000	$ 10,228,000	$ 3,983,000	$ 3,542,000	$ 2,750,000	$ 31,585,000	$ 20,502,000	$ 10,165,000
General and administrative	3,453,000	3,019,000	3,324,000	2,776,000	3,569,000	2,025,000	677,000	730,000	12,572,000	7,001,000	2,055,000
Other (expense) income, net	4,502,000	4,925,000	2,946,000	(5,518,000)	51,000	(2,203,000)	(2,000)	8,000	6,855,000	(2,146,000)	22,000
Loss before (benefit from) provision for income taxes	(8,575,000)	(5,939,000)	(8,430,000)	(14,358,000)	(13,746,000)	(8,211,000)	(4,221,000)	(3,472,000)	(37,302,000)	(29,649,000)	(12,198,000)
(Benefit from) provision for income taxes	(189,000)	83,000	(334,000)		37,000				(440,000)	37,000	0
Net loss	(8,386,000)	(6,022,000)	(8,096,000)	(14,358,000)	(13,783,000)	(8,211,000)	(4,221,000)	(3,472,000)	$ (36,862,000)	$ (29,686,000)	$ (12,198,000)
Other comprehensive income (loss)	$ (3,788,000)	$ (4,329,000)	$ (2,681,000)	$ 6,346,000	$ 418,000	$ 1,848,000	$ 1,579,000	$ 143,000
Weighted average ordinary shares outstanding basic and diluted	32,762	28,139	28,053	27,862	27,336	21,514	23,688	11,680	29,176,000	18,187,000	8,954,000
Basic and diluted net loss per ordinary share	$ (0.26)	$ (0.21)	$ (0.29)	$ (0.52)	$ (0.50)	$ (0.38)	$ (0.18)	$ (0.30)	$ (1.26)	$ (1.63)	$ (1.36)

Subsequent Events - Additional

Subsequent Events - Additional Information (Details)	Mar. 04, 2019£ / shares	Mar. 04, 2019$ / shares	Dec. 31, 2018£ / shares	Dec. 31, 2017£ / shares
Subsequent Event [Line Items]
Ordinary share, par value per share			£ 0.01	£ 0.01
Subsequent Event | Tungsten Bidco Limited
Subsequent Event [Line Items]
Share, price per ordinary share | $ / shares		$ 25.50
Ordinary share, par value per share	£ 0.01
Minimum percentage of shares held by shareholders in value and votes cast, eligible for approval of acquisition	75.00%