Biohaven Pharmaceutical Holding 10-K 28 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Feb. 26, 2019	Jun. 29, 2018
Document and Entity Information
Entity Registrant Name	Biohaven Pharmaceutical Holding Co Ltd.
Entity Central Index Key	1,689,813
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Shell Company	false
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Public Float			$ 1,585
Entity Common Stock, Shares Outstanding		44,262,658

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash	$ 264,249	$ 131,468
Prepaid expenses and other current assets	8,090	5,197
Total current assets	272,339	136,665
Property and equipment, net	6,248	2,344
Equity method investment	11,414	7,847
Other assets	11	32
Total assets	290,012	146,888
Current liabilities:
Accounts payable	10,752	4,721
Accrued expenses	8,782	4,708
Total current liabilities	19,534	9,429
Non-recourse debt related to sale of future royalties	117,515	0
Fair value of warrant liability	0	4,021
Other long-term liabilities	2,043	1,467
Total liabilities	139,092	14,917
Commitments and contingencies
Shareholders' equity:
Common shares, no par value; 200,000,000 authorized as of December 31, 2018 and 2017; 44,197,549 and 36,057,748 shares issued and outstanding as of December 31, 2018 and December 31, 2017, respectively	554,384	311,061
Additional paid-in capital	40,104	23,556
Accumulated deficit	(443,568)	(202,646)
Total shareholders' equity	150,920	131,971
Total liabilities, convertible preferred shares and shareholders' equity	$ 290,012	$ 146,888

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Common stock, par value (in dollars per share)	$ 0	$ 0
Common stock, authorized (shares)	200,000,000	200,000,000
Common stock, issued (shares)	44,197,549	36,057,748
Common stock, outstanding (shares)	44,197,549	36,057,748

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - 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
Operating expenses:
Research and development	$ 37,958	$ 47,362	$ 29,052	$ 75,579	$ 22,686	$ 34,996	$ 21,019	$ 10,740	$ 189,951	$ 89,441	$ 55,529
General and administrative	10,108	7,574	9,064	7,857	5,614	4,571	4,199	3,757	34,603	18,141	5,109
Total operating expenses	48,066	54,936	38,116	83,436	28,300	39,567	25,218	14,497	224,554	107,582	60,638
Loss from operations	(48,066)	(54,936)	(38,116)	(83,436)	(28,300)	(39,567)	(25,218)	(14,497)	(224,554)	(107,582)	(60,638)
Other income (expense):
Interest expense	(5)	(12)	(13)	(8)	0	(239)	(362)	(305)	(38)	(906)	(385)
Non-cash interest expense on liability related to sale of future royalties	(5,592)	(5,633)	(501)	0					(11,726)	0	0
Change in fair value of warrant liability	0	0	0	(1,182)	2,268	(2,426)	(2,629)	(454)	(1,182)	(3,241)	154
Change in fair value of derivative liability					0	0	223	289	0	512	(65)
Change in fair value of contingent equity liability					0	0	(9,707)	(3,375)	0	(13,082)	(2,263)
Loss from equity method investment	(742)	(697)	(641)	(728)	(681)	(638)	(348)	(218)	(2,808)	(1,885)	(247)
Other	(151)	(2)	27	(21)					(147)	0	0
Total other income (expense), net	(6,490)	(6,344)	(1,128)	(1,939)	1,577	(3,293)	(12,823)	(4,063)	(15,901)	(18,602)	(2,806)
Loss before provision for income taxes	(54,556)	(61,280)	(39,244)	(85,375)	(26,723)	(42,860)	(38,041)	(18,560)	(240,455)	(126,184)	(63,444)
Provision for income taxes	194	161	25	87	359	55	399	193	467	1,006	90
Net loss and comprehensive loss					(27,082)	(42,915)	(38,440)	(18,753)	(240,922)	(127,190)	(63,534)
Net loss attributable to non-controlling interests									0	0	(143)
Accretion of beneficial conversion feature on Series A preferred shares					0	0	(8,006)	(4,000)	0	(12,006)	0
Net loss attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd.	$ (54,750)	$ (61,441)	$ (39,269)	$ (85,462)	$ (27,082)	$ (42,915)	$ (46,446)	$ (22,753)	$ (240,922)	$ (139,196)	$ (63,677)
Net loss per share attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd. - basic and diluted	$ (1.34)	$ (1.53)	$ (1.01)	$ (2.32)	$ (0.75)	$ (1.19)	$ (1.78)	$ (1.74)	$ (6.15)	$ (5)	$ (5.05)
Weighted average common shares outstanding - basic and diluted	40,938,709	40,147,735	38,942,545	36,793,090	35,984,111	35,930,698	26,038,192	13,088,861	39,188,458	27,845,576	12,608,366

CONSOLIDATED STATEMENT OF CONVE

CONSOLIDATED STATEMENT OF CONVERTIBLE PREFERRED SHARES AND SHAREHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Total Biohaven Pharmaceutical Holding Company Ltd. Shareholders' Equity	Common Shares	Additional Paid-in Capital	Accumulated Deficit	Non-Controlling Interests	Series A Convertible Preferred Shares
Balance, beginning of period at Dec. 31, 2015							$ 0
Balance, beginning of period (in shares) at Dec. 31, 2015							0
Increase (Decrease) in Temporary Equity
Issuance of Series A convertible preferred shares, net of cash offering costs							$ 38,270
Issuance of Series A convertible preferred shares, net of cash offering costs (in shares)							4,305,209
Issuance of Series A convertible preferred shares as payment of related offering costs (in shares)							105,010
Issuance of Series A convertible preferred shares in settlement of contingent equity liability							$ 5,000
Issuance of Series A convertible preferred shares in settlement of contingent equity liability (in shares)							538,150
Balance, end of period at Dec. 31, 2016							$ 43,270
Balance, end of period (in shares) at Dec. 31, 2016							4,948,369
Balances at Dec. 31, 2015	$ 1,087	$ 1,144	$ 8,665	$ 4,258	$ (11,779)	$ (57)
Balances (in shares) at Dec. 31, 2015			11,569,000
Increase (Decrease) in Stockholders' Equity
Issuance of common shares, net of offering costs	11,279	11,279	$ 11,279
Issuance of common shares, net of offering costs (in shares)			1,519,500
Issuance of common share warrant in connection with license agreement	2,127	2,127		2,127
Beneficial conversion feature on Series A convertible preferred shares	0
Accretion of beneficial conversion feature on Series A convertible preferred shares	0
Issuance of common shares as payment of equity investment	0
Acquisition of BPI	(595)	(509)		(509)		(86)
Non-cash share-based compensation expense	4,603	4,603		4,603
Net loss	(63,534)	(63,677)			(63,677)	143
Balances at Dec. 31, 2016	(45,033)	(45,033)	$ 19,944	10,479	(75,456)	0
Balances (in shares) at Dec. 31, 2016			13,088,500
Increase (Decrease) in Temporary Equity
Issuance of Series A convertible preferred shares, net of cash offering costs							$ 38,666
Issuance of Series A convertible preferred shares, net of cash offering costs (in shares)							4,305,182
Issuance of Series A convertible preferred shares as payment of related offering costs (in shares)							105,009
Conversion of Series A convertible preferred shares to common shares							$ (81,936)
Conversion of Series A convertible preferred shares to common shares (in shares)							(9,358,560)
Balance, end of period at Dec. 31, 2017							$ 0
Balance, end of period (in shares) at Dec. 31, 2017							0
Increase (Decrease) in Stockholders' Equity
Issuance of common shares, net of offering costs	176,128	176,128	$ 176,128
Issuance of common shares, net of offering costs (in shares)			11,385,000
Beneficial conversion feature on Series A convertible preferred shares	12,006	12,006		12,006			$ (12,006)
Accretion of beneficial conversion feature on Series A convertible preferred shares	(12,006)	(12,006)		(12,006)			12,006
Issuance of common shares as payment of equity investment	352	352	$ 352
Issuance of common shares as payment for equity investment (in shares)			32,500
Conversion of Series A convertible preferred shares to common shares	81,936	81,936	$ 81,936
Conversion of Series A convertible preferred shares to common shares (shares)			9,358,560
Issuance of common shares in settlement of contingent equity liability	32,020	32,020	$ 32,020
Issuance of common shares in settlement of contingent equity liability (in shares)			1,883,523
Issuance of common share warrant as consideration for services	93	93		93
Exercise of stock options	426	426	$ 681	(255)
Exercise of stock options (in shares)			(309,665)
Non-cash share-based compensation expense	13,239	13,239		13,239
Net loss	(127,190)	(127,190)			(127,190)
Balances at Dec. 31, 2017	131,971	131,971	$ 311,061	23,556	(202,646)	0
Balances (in shares) at Dec. 31, 2017			36,057,748
Balance, end of period at Dec. 31, 2018							$ 0
Balance, end of period (in shares) at Dec. 31, 2018							0
Increase (Decrease) in Stockholders' Equity
Issuance of common shares as payment for license agreement	4,080	4,080	$ 4,080
Issuance of common shares as payment for license agreement (shares)			109,523
Issuance of common shares, net of offering costs	230,339	230,339	$ 230,339
Issuance of common shares, net of offering costs (in shares)			6,970,171
Beneficial conversion feature on Series A convertible preferred shares	0
Accretion of beneficial conversion feature on Series A convertible preferred shares	0
Issuance of common shares as payment of equity investment	0
Exercise of ALS Biopharma warrants, net of settlement shares			489,359
Reclassification of warrant liability to equity	5,203	5,203		5,203
Exercise of stock options	3,324	3,324	$ 8,904	(5,580)
Exercise of stock options (in shares)			(570,748)
Non-cash share-based compensation expense	16,925	16,925		16,925
Net loss	(240,922)	(240,922)			(240,922)
Balances at Dec. 31, 2018	$ 150,920	$ 150,920	$ 554,384	$ 40,104	$ (443,568)	$ 0
Balances (in shares) at Dec. 31, 2018			44,197,549

CONSOLIDATED STATEMENT OF CON_2

CONSOLIDATED STATEMENT OF CONVERTIBLE PREFERRED SHARES AND SHAREHOLDERS' EQUITY (DEFICIT) (PARENTHETICAL) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Common shares
Issuance of common stock, issuance costs		$ 120
Series A Convertible Preferred Shares
Issuance of common stock, issuance costs	$ 1,334	$ 1,730

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	$ (240,922)	$ (127,190)	$ (63,534)
Adjustments to reconcile net loss to net cash used in operating activities:
Non-cash share-based compensation expense	16,925	13,239	4,603
Non-cash interest expense on non-recourse debt related to sale of future royalties	11,726	0	0
Issuance of common shares as payment for license agreement	4,080	0	0
Non-cash interest expense	0	784	374
Fair value of contingent equity liability under license agreements	0	0	21,675
Fair value of warrants issued as consideration for license agreement	0	0	2,127
Change in fair value of warrant liability	1,182	3,241	(154)
Change in fair value of derivative liability	0	(512)	65
Change in fair value of contingent equity liability	0	13,082	2,263
Loss from equity method investment	2,808	1,885	247
Deferred tax assets	0	9	0
Other non-cash items	269	64	(4)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,893)	(4,730)	24
Other assets	21	(32)	0
Accounts payable	5,390	3,975	678
Accrued expenses	3,697	1,341	2,148
Other long-term liabilities	576	29	(16)
Net cash used in operating activities	(197,141)	(94,815)	(29,504)
Cash flows from investing activities:
Purchases of property and equipment	(4,165)	(541)	(26)
Purchase of equity method investment	(6,375)	(6,627)	(3,000)
Net cash used in investing activities	(10,540)	(7,168)	(3,026)
Cash flows from financing activities:
Proceeds from issuance of common shares	190,125	179,996	11,399
Proceeds from the sale of future royalties	106,047	0	0
Proceeds from issuance of common stock related to sale of future royalties	43,953	0	0
Proceeds from issuance of Series A preferred shares	0	40,000	40,000
Proceeds from borrowings	0	0	5,000
Proceeds from exercise of stock options	3,324	426	0
Payments of related party notes payable	0	(595)	0
Repayment of notes payable	0	(5,000)	0
Payments of offering costs	(2,987)	(5,068)	(1,507)
Payments of debt issuance costs	0	0	(197)
Advanced payment for the second closing of Series A preferred stock	0	0	67
Net cash provided by financing activities	340,462	209,759	54,762
Net increase in cash	132,781	107,776	22,232
Cash at beginning of period	131,468	23,692	1,460
Cash at end of period	264,249	131,468	23,692
Supplemental disclosure of cash flow information:
Cash paid for interest	0	122	11
Cash paid for income taxes	333	1,049	0
Supplemental disclosure of non-cash investing and financing activities:
Deferred offering costs included in accrued expenses	1,018	0	134
Series A convertible preferred share offering costs included in accrued expenses	0	0	343
Issuance of warrants to guarantor and co-guarantor of notes payable	0	0	934
Beneficial conversion feature on Series A preferred shares	0	12,006	0
Accretion of beneficial conversion feature on Series A preferred shares	0	12,006	0
Issuance of Series A preferred shares as payment of offering costs	0	1,242	975
Issuance of Series A preferred shares in settlement of contingent equity liability	0	0	5,000
Issuance of common shares as payment of equity investment	0	352	0
Issuance of notes payable to related parties in connection with acquisition of BPI	0	0	595
Purchases of property and equipment under financing lease	$ 0	$ 1,787	$ 0

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	Nature of the Business and Basis of Presentation Biohaven Pharmaceutical Holding Company Ltd. (“we,” “us” or the “Company”) was incorporated in Tortola, British Virgin Islands in September 2013. We are a clinical-stage biopharmaceutical company with a portfolio of innovative, late-stage product candidates targeting neurological diseases, including rare disorders. Our product candidates are based on multiple mechanisms —calcitonin gene-related peptide (“CGRP”) receptor antagonists, glutamate modulators and myeloperoxidase ("MPO") inhibitor—which we believe have the potential to significantly alter existing treatment approaches across a diverse set of neurological indications with high unmet need in both large and orphan indications. The most advanced product candidate from the Company’s CGRP receptor antagonist platform is rimegepant, which the Company is developing for the acute and preventive treatment of migraine and for which it has completed three Phase 3 clinical trials in acute treatment of migraine. 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, compliance with government regulations 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 may require additional capital, additional personnel and infrastructure, and further regulatory and other capabilities. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize revenue from product sales. The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of the Company and its subsidiaries after elimination of all intercompany accounts and transactions. Investments in companies in which the Company owns less than a 50% equity interest and where it has the ability to exercise significant influence over the operating and financial policies of the investee are accounted for using the equity method of accounting. Biohaven Pharmaceuticals, Inc. The Company has historically outsourced all of the research and clinical development for its programs under a master services agreement (the "MSA") with Biohaven Pharmaceuticals, Inc. ("BPI"). BPI was incorporated in the state of Delaware in July 2013. The three founders of BPI, each of whom owned 33.33% of the equity of BPI through December 31, 2016, are related parties of the Company (see Note 18). Substantially all of the operations of BPI have been performed in service to the Company under the terms of the MSA, and substantially all of the revenue for the operations of BPI was provided by us. From inception, the Company has consolidated the results of BPI. On December 31, 2016, the Company acquired 100% of the issued and outstanding shares of BPI (see Note 18). From inception through the acquisition of BPI, 100% of the equity in BPI was reflected as a net loss attributable to non-controlling interest on the consolidated statement of operations and comprehensive loss. Since the acquisition of BPI on December 31, 2016, the Company no longer reports any non-controlling interest related to BPI. Stock Split In October 2016, the Company effected a 500-for-one stock split of its issued and outstanding common shares. Accordingly, all share and per share amounts for all periods presented in the accompanying consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this stock split. Initial Public Offering On May 3, 2017, the Company's registration statement on Form S-1 relating to its initial public offering of its common shares (the "IPO") was declared effective by the Securities and Exchange Commission ("SEC"). The IPO closed on May 9, 2017 and the Company issued and sold 9,900,000 common shares at a public offering price of $17.00 per share for net proceeds of $152,651 after deducting underwriting discounts and commissions of $11,781 and other offering expenses of approximately $3,868. Upon the closing of the IPO, all convertible preferred shares then outstanding converted into an aggregate of 9,358,560 common shares. In addition, on May 9, 2017, the underwriters of the IPO fully exercised their option to purchase additional shares, and on May 11, 2017, the Company issued and sold 1,485,000 common shares for net proceeds of $23,478 after deducting underwriting discounts and commissions of $1,767. Thus, the aggregate net proceeds to the Company from the IPO, after deducting underwriting discounts and commissions and other offering costs, were $176,128. In connection with the completion of its IPO, the Company issued an aggregate of 1,883,523 common shares to Bristol Myers-Squibb Company ("BMS") and AstraZeneca AB ("AstraZeneca") in satisfaction of obligations to contingently issue equity securities pursuant to the license agreements (see Note 13) for no additional consideration. Also in connection with the completion of its IPO in May 2017, the Company amended its memorandum and articles of association to authorize the issuance of up to 200,000,000 no par value common shares and 10,000,000 no par value undesignated preferred shares. Going Concern In accordance with Accounting Standards Update ("ASU") 2014-15, Disclosure of Uncertainties about an Entity's Ability to Continue as a Going Concern (Subtopic 205-40) , the Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company's ability to continue as a going concern within one year after the date that the consolidated financial statements are issued. In June 2018, the Company entered into a funding agreement (the "Funding Agreement") to sell tiered, sales-based royalty rights on global net sales of pharmaceutical products containing the compounds rimegepant or BHV-3500 and certain derivative compounds thereof ("Products") to RPI Finance Trust ("RPI") in exchange for $100,000 in cash (see Note 8). Also in June 2018, in connection with the Funding Agreement, the Company entered into a common stock purchase agreement ("Purchase Agreement") with RPI, pursuant to which the Company, in a private placement, issued and sold 1,111,111 common shares of the Company to RPI. RPI paid the Company $45.00 per share for gross proceeds of $50,000,000 (see Note 11). The aggregate net proceeds to the Company from the transactions with RPI, after deducting issuance costs of $377, was $149,623. In December 2018, we closed on an underwritten public offering of 3,859,060 of common shares, including the full exercise of the underwriters' option to purchase additional shares, at a price to the public of $37.25 per share. The aggregate net proceeds to us from the offering, after deducting the underwriting discounts and commissions and offering expenses payable, were approximately $134,485. Through December 31, 2018, the Company has funded its operations primarily with proceeds from sales of preferred and common shares and proceeds from the IPO. The Company has incurred recurring losses since its inception, including net losses of $240,922, $127,190 and $63,534 during the years ended December 31, 2018, 2017 and 2016, respectively. In addition, as of December 31, 2018, the Company had an accumulated deficit of $443,568. The Company expects to continue to generate operating losses for the foreseeable future. As of February 28, 2019, the issuance date of these consolidated financial statements, the Company expects that its cash of $264,249 as of December 31, 2018 will be sufficient to fund operating expenses, financial commitments and other cash requirements through at least one year after the issuance date of these financial statements. The future viability of the Company beyond that point is dependent on its ability to raise additional capital to finance its operations. To execute its business plans, the Company will require funding to support its continuing operations and pursue its growth strategy. Until such time as the Company can generate significant revenue from product sales, if ever, it expects to finance its operations through the sale of public or private equity, debt financings or other capital sources, including collaborations with other companies or other strategic transactions. The Company may not be able to obtain financing on acceptable terms, or at all. The terms of any financing may adversely affect the holdings or the rights of the Company's shareholders. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations. Although management continues to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if 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	Summary of Significant Accounting Policies Use of Estimates The preparation of consolidated financial statements in conformity with 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 and the valuation of common shares, stock options, warrants, derivative instruments, contingent equity instruments, and non-cash interest expense on liability related to sale of future royalties. In addition, management’s assessment of the Company’s ability to continue as a going concern involves the estimation of the amount and timing of future cash inflows and outflows. 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 from those estimates. Equity Method Investments, Including Related Impairment Investments in non-public companies in which the Company owns less than a 50% equity interest and where it has the ability to exercise significant influence over the operating and financial policies of the investee are accounted for using the equity method of accounting. The Company's proportionate share of the net income or loss of the equity method investment is included in other income (expense), net in the consolidated statement of operations and comprehensive loss and results in a corresponding adjustment to the carrying value of the investment on the consolidated balance sheet. Dividends received reduce the carrying value of the investment. An assessment of whether or not we have the power to direct activities that most significantly impact Kleo’s economic performance and to identify the party that obtains the majority of the benefits of the investment was performed as of December 31, 2018 and December 31, 2017, and will be performed as of each subsequent reporting date. After each of these assessments, we concluded that the activities that most significantly impact Kleo’s economic performance are the ability to direct its research activities, the ability to select vendors to perform the research, the ability to maintain research staff and the ability to raise additional funds, each of which are directed by Kleo. Based on the outcome of these assessments, we concluded that our investment in Kleo should be accounted for under the equity method. Changes related to this assessment could have a material impact on our financial statements. We also periodically review the carrying value of our investment in Kleo to determine if there has been an other-than-temporary decline in carrying value. A variety of factors are considered when determining if a decline in carrying value is other than temporary, including, among other factors, Kleo’s financial condition and business prospects, as well as our intent with regard to the investment. Changes related to the analysis of impairment of our investment in Kleo could have a material impact on our financial statements. Property and Equipment Property and equipment are recorded at cost and depreciated or amortized using the straight-line method over the estimated useful lives of the respective assets. As of December 31, 2018, the Company's property and equipment consisted of an office building, office equipment and computer equipment. As of December 31, 2017, the Company's property and equipment consisted of office equipment and computer equipment, as well as construction in progress comprised of computer software and leasehold improvements. The fixed assets have the following useful lives: Building 30 years Office equipment 3 - 5 years Computer software 3 - 5 years Computer equipment 3 years 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 loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred. Property and equipment are monitored regularly for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Fair Value Measurements Certain assets of the Company are carried at fair value under GAAP. 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. The Company's warrant liability, derivative liability and contingent equity liability are carried at fair value, based upon Level 3 inputs described above (see Note 3). The carrying values of other current assets, accounts payable and accrued expenses approximate their fair values due to the short-term nature of these assets and liabilities. Segment Information The Company manages its operations as a single segment, the development of therapies targeting neurological diseases, for the purposes of assessing performance and making operating decisions. All of the Company's tangible assets are held in the 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, non-cash share-based compensation and benefits, third-party license fees, and external costs of vendors engaged to conduct clinical development activities and clinical trials as well as to manufacture clinical trial materials. Non-refundable prepayments for goods or services that will be used or rendered for future research and development activities are deferred and capitalized. Such amounts are recognized as an expense as the goods are delivered or the related services are performed, or until it is no longer expected that the goods will be delivered or the services rendered. The Company has entered into various research and development-related contracts. These agreements are 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. Certain judgments and 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. Non-Cash Interest Expense on Liability Related to Sale of Future Royalties The Company accounted for the Funding Agreement with RPI as a liability financing, primarily because it has significant continuing involvement in generating the future revenue on which the royalties are based. The debt will be amortized under the effective interest rate method and, accordingly, the Company is recognizing non-cash interest expense over the estimated term of the Funding Agreement. The liability related to sale of future royalties, and the debt amortization, are based on the Company's current estimate of future royalties expected to be paid over the estimated term of the Funding Agreement. The Company will periodically assess the expected royalty payments and, if materially different than its previous estimate, will prospectively adjust and recognize the related non-cash interest expense. The transaction costs associated with the liability will be amortized to non-cash interest expense over the estimated term of the Funding Agreement. Non-Cash Share-Based Compensation The Company measures stock options granted to employees, non-employees, and directors based on the fair value on the date of the grant and recognizes non-cash compensation expense of those awards, over the requisite service period, which is generally the vesting period of the respective award. Forfeitures are accounted for as they occur. Generally, the Company issues stock options with only service-based vesting conditions and records the expense for these awards using the straight-line method. The Company also issues, from time to time, stock options with performance-based vesting conditions and records the expense for these awards when the Company concludes that it is probable that the performance condition will be achieved. Effective July 1, 2018, the Company adopted Accounting Standards Update ("ASU") No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Non-employee Share-based Payment Accounting ("ASU 2018-07"), which sets out to simplify the accounting for non-employee share-based awards. The ASU expands the scope of Topic 718, Compensation-Stock Compensation, which currently only includes share-based payments issued to employees, to also include share-based payments issued to non-employees for goods and services. Consequently, the accounting for share-based payments to non-employees and employees is substantially aligned. ASU 2018-07 impacts the value at which share-based payments to non-employees is recognized. Prior to the adoption of ASU 2018-07 for share-based awards granted to non-employees, including consultants, non-cash compensation expense was recognized over the period during which services were rendered by such non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of the unvested awards were remeasured using the then-current fair value of the Company's common shares and updated assumption inputs in the Black-Scholes option-pricing model. After adoption of ASU 2018-07, the measurement date for non-employee awards is the date of the grant. The non-cash compensation expense for non-employees is recognized, without changes in the fair value of the award, over the requisite service period, which is the vesting period of the respective award. The non-cash compensation expense for non-employees was measured as of the adoption date of July 1, 2018, and this amount is the basis for prospective expense recognition. All of the Company's non-employee awards were previously measured as of June 30, 2018. Accordingly, no cumulative adjustment to beginning retained earnings was recorded as a result of the ASU 2018-07 adoption, as the measured value prior to adoption and the remeasured value on the date of adoption were materially the same. The Company classifies non-cash share-based compensation expense in its consolidated statement of operations and comprehensive loss in the same manner in which the award recipient's payroll costs are classified or in which the award recipient's service payments are classified. The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. The Company lacks a sufficient history of company-specific historical and implied volatility information for its shares. Therefore, it estimates its expected share price volatility based on the historical volatility of publicly traded peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded share price. The expected term of the Company's stock options has been determined utilizing the "simplified" method for awards that qualify as "plain-vanilla" options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends on common shares and does not expect to pay any cash dividends in the foreseeable future. Warrant Liability In connection with entering into a credit agreement, the Company issued warrants to purchase common shares to two of the Company's directors in connection with a guarantee of its obligations under the agreement (see Note 9). The Company classifies the warrants as a liability on its consolidated balance sheet because each warrant represents a freestanding financial instrument that is not indexed to the Company's own shares. The warrant liability was initially recorded at fair value upon entering into the credit agreement and is subsequently remeasured to fair value at each reporting date. Changes in the fair value of the warrant liability are recognized as a component of other income (expense), net in the consolidated statement of operations and comprehensive loss. Changes in the fair value of the warrant liability will continue to be recognized until the warrants are exercised, expire or qualify for equity classification. Changes in the fair value of the warrant liability, until expiration of the anti-dilution price protection provisions, are recognized as a component of other income (expense), net, in the Company’s consolidated statement of operations and comprehensive loss. Upon expiration of the provision, the Company discontinued classification of these warrants as a liability, and has accordingly reclassified the fair value to additional paid-in capital within shareholders’ equity. 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 the Company's 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 its deferred tax assets will be recovered from future taxable income and, to the extent it 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 provision for income taxes includes the effects of applicable tax reserves, or unrecognized tax benefits, as well as the related net interest and penalties. Net Income (Loss) per Share The Company follows the two-class method when computing net income (loss) per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common shareholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. Net income (loss) per share attributable to common shareholders is calculated based on net income (loss) attributable to Biohaven Pharmaceutical Holding Company Ltd. and excludes net income (loss) attributable to non-controlling interests for relevant periods. Basic net income (loss) per share attributable to common shareholders is computed by dividing the net income (loss) attributable to common shareholders by the weighted average number of common shares outstanding for the period. Diluted net income (loss) attributable to common shareholders is computed by adjusting net income (loss) attributable to common shareholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common shareholders is computed by dividing the diluted net income (loss) attributable to common shareholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares. For purpose of this calculation, outstanding options, warrants to purchase common shares, convertible preferred shares and contingently issuable equity are considered potential dilutive common shares. The Company's convertible preferred shares contractually entitled the holders of such shares to participate in dividends but contractually did not require the holders of such shares to participate in losses of the Company. In periods in which the Company reports a net loss attributable to common shareholders, diluted net loss per share attributable to common shareholders is the same as basic net loss per share attributable to common shareholders, since potentially dilutive common shares are considered to be anti-dilutive. Recently Adopted Accounting Pronouncements Effective January 1, 2018, the Company adopted ASU No. 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017-09”), which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. The adoption of this new guidance had no impact on the Company's financial position or operating results. Effective January 1, 2018, the Company adopted ASU 2016-18, “Statement of Cash Flows (Topic 230): Restricted Cash,” which requires entities to show the change in the total of cash, cash equivalents, restricted cash and restricted cash equivalents within the statement of cash flows. The guidance is effective retrospectively. As a result, the Company retrospectively included restricted cash in the beginning cash for the twelve months ended December 31, 2017 on the consolidated statement of cash flows, and no longer separately presents transfers between unrestricted cash and restricted cash. The Company did not have restricted cash as of December 31, 2018 or December 31, 2017. Effective January 1, 2018, the Company adopted ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“ASU 2016-15”), which addresses diversity in practice in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. The adoption of this new guidance had no impact on the Company's financial position or operating results. Effective July 1, 2018, the Company adopted Accounting Standards Update ("ASU") No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Non-employee Share-based Payment Accounting ("ASU 2018-07"), which sets out to simplify the accounting for non-employee share-based awards. The ASU expands the scope of Topic 718, Compensation-Stock Compensation, which currently only includes share-based payments issued to employees, to also include share-based payments issued to non-employees for goods and services. Consequently, the accounting for share-based payments to non-employees and employees is substantially aligned. ASU 2018-07 impacts the value at which share-based payments to non-employees is recognized. Prior to the adoption of ASU 2018-07 for share-based awards granted to non-employees, including consultants, non-cash compensation expense was recognized over the period during which services were rendered by such non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of the unvested awards were remeasured using the then-current fair value of the Company's common shares and updated assumption inputs in the Black-Scholes option-pricing model. After adoption of ASU 2018-07, the measurement date for non-employee awards is the date of the grant. The non-cash compensation expense for non-employees is recognized, without changes in the fair value of the award, over the requisite service period, which is the vesting period of the respective award. The non-cash compensation expense for non-employees was measured as of the adoption date of July 1, 2018, and this amount is the basis for prospective expense recognition. All of the Company's non-employee awards were previously measured as of June 30, 2018. Accordingly, no cumulative adjustment to beginning retained earnings was recorded as a result of the ASU 2018-07 adoption, as the measured value prior to adoption and the remeasured value on the date of adoption were materially the same. Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease, respectively. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. ASU 2016-02 (Accounting Standards Codification (“ASC”) Topic 842) supersedes the previous leases standard, ASC 840, Leases. The standard is effective for public entities for annual periods beginning after December 15, 2018 and for interim periods within those fiscal years. Subsequently, in July of 2018, the FASB issued ASU No. 2018-10, Codification Improvements to Topic 842, Leases (“ASU 2018-10”), and ASU No. 2018-11, Leases (Topic 842): Targeted Improvements (“ASU 2018-11”), both of which clarify and enhance the certain amendments made in ASU 2016-02 and will be adopted in conjunction with ASU 2016-02. The Company intends to elect the package of practical expedients and is currently evaluating the impact that the adoption of ASU 2016-02 will have on its 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 of Financial Assets and Liabilities	Fair Value of Financial Assets and Liabilities The following table presents information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values: Fair Value Measurement as of December 31, 2017 Using: Level 1 Level 2 Level 3 Total Liabilities: Warrant liability $ — $ — $ 4,021 $ 4,021 $ — $ — $ 4,021 $ 4,021 The Company held no financial assets or liabilities measured at fair value on a recurring basis as of December 31, 2018. Valuation of Warrant Liability The warrant liability in the table above is composed of the fair value of warrants to purchase common shares that the Company issued to two of its directors in connection with a guarantee of its obligations under a credit agreement (see Note 9). On January 26, 2018, the anti-dilution price protection provisions contained within the warrants expired. Due to the expiration of these provisions, the Company discontinued classification of these warrants as a liability, and has accordingly reclassified them to additional paid-in capital within shareholders' equity. On expiration, the fair value of the warrant liability was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The Company utilized the Black-Scholes option pricing model to value the warrant liability. The Black-Scholes option pricing model incorporated assumptions and estimates to value the warrant liability. Estimates and assumptions impacting the fair value measurement included the number of shares for which the warrants will be exercisable, the fair value per share of the underlying common shares issuable upon exercise of the warrants, the remaining contractual term of the warrants, the risk-free interest rate, the expected dividend yield, and the expected volatility of the price of the underlying common shares. The fair value per share of the Company’s common shares was based on the closing trading price of the shares on January 26, 2018, the day of expiration, and the increase in the fair value of the common shares during the time period from December 31, 2017 to expiration is the primary reason for the increase in the fair value of the warrant liability during the same period. The Company was a private company prior to its IPO in May 2017 and therefore lacks company-specific historical and implied volatility information of its shares. Therefore, it estimated its expected share volatility based on the historical volatility of publicly traded peer companies for a term equal to the remaining contractual term of the warrants. The risk-free interest rate was determined by reference to the U.S. Treasury yield curve for time periods approximately equal to the remaining contractual term of the warrants. The Company estimated a 0% expected dividend yield based on the fact that the Company has never paid or declared dividends and does not intend to do so in the foreseeable future. Valuation of Derivative Liability The fair value of the derivative liability recognized in connection with the Company's license agreement with Yale (see Note 13) was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of the derivative liability was determined using a Monte-Carlo simulation, which is a statistical method used to generate a defined number of share price paths to develop a reasonable estimate of the range of the expected share prices. The Monte-Carlo simulation incorporated assumptions and estimates to value the derivative liability, including the amount of the payment, the settlement date, the trading price of the Company’s common shares, the risk-free interest rate and the expected volatility of the price of the underlying common shares. In April 2017, the agreement with Yale was amended such that if the change-of-control event was an IPO, the change-of-control payment would be due to Yale on the first trading day when Yale was free to sell its equity interest in the Company and the change-of-control fee would be reduced by the dollar value of Yale’s equity interest in the Company on the first trading day when Yale was free to sell its equity interest in the Company. Yale’s equity interest in the Company was subject to a lock-up agreement, which generally restricted Yale’s shares from being traded until October 31, 2017 and accordingly, the amount due to Yale in connection with the change-of-control provision of the agreement, if any, would be determined upon expiration of the lock-up period. The Company continued to remeasure the derivative liability to fair value at each reporting date and recognized any changes in the fair value of the derivative liability through October 31, 2017. The derivative liability as of December 31, 2018, and upon expiration of the lock-up period was determined to be $0 based on the value of the Company’s shares on this date. Valuation of Contingent Equity Liability BMS. The fair value of the contingent equity liability recognized in connection with the Company's license agreement with BMS (see Note 13) was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of the contingent equity liability was determined using the probability-weighted expected returns method ("PWERM"), which considered as inputs the probability of occurrence of events that would trigger the issuance of shares, the expected timing of such events, the value of the contingently issuable equity and a risk-adjusted discount rate. As of December 31, 2016, the assumed probability of occurrence of the event that was most probable of triggering the issuance of shares was 75%, the expected timing of such an event was estimated to be less than one year, the value of the contingently issuable equity was $18,750 and the discount rate was assessed to be 0%. In connection with the closing of the IPO in May 2017, the conditions for issuing shares in connection with the contingent equity liability were satisfied, and accordingly, the Company issued 1,345,374 common shares to BMS. The contingent equity liability was adjusted to fair value immediately prior to the completion of the IPO, and upon issuance of the common shares, the contingent equity liability was reclassified to equity. AstraZeneca. The fair value of the contingent equity liability recognized in connection with the Company's license agreement with AstraZeneca (see Note 13) was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of the contingent equity liability was determined using the PWERM, which considered as inputs the probability of occurrence of events that would trigger the issuance of shares, the expected timing of such events, the value of the contingently issuable equity and a risk-adjusted discount rate. The contingently issuable equity is issuable in two tranches, each for a fixed dollar amount of $5,000, for a total amount of $10,000. Using the PWERM, the Company assessed the fair value of each tranche of the contingent equity liability separately. In October 2016, upon completion of the Series A First Closing (see Note 10), the first tranche of contingently issuable equity became issuable to AstraZeneca. As a result, the Company issued to AstraZeneca 538,150 Series A preferred shares with an aggregate fair value of $5,000, or $9.2911 per share, in satisfaction of the obligation to issue the first tranche of equity under the agreement. Upon the issuance of the 538,150 Series A preferred shares to AstraZeneca in October 2016, the Company reclassified the carrying value of the first tranche contingent equity liability, equal to the then-current fair value of $5,000, to the carrying value of Series A preferred shares. The shares related to the second tranche became issuable upon the earlier of (i) the initiation of a Phase 2b or equivalent clinical trial of a product candidate based on the licensed patent rights and (ii) any liquidity event, including an IPO, any change of control or any assignment of the Company's rights or obligations under the license agreement. As of December 31, 2016, the Company determined that the fair value of the second tranche contingent equity liability was $4,875. In determining this fair value, the assumed probability of occurrence of the event that was most probable of triggering the issuance of shares was 65%, the expected timing of such an event was estimated to be less than one year, the value of the contingently issuable equity was $7,500 and the discount rate was assessed to be 0%. In connection with the closing of the IPO in May 2017, the conditions for issuing shares in connection with the contingent equity liability were satisfied, and accordingly, the Company issued 538,149 common shares to AstraZeneca. The contingent equity liability was adjusted to fair value immediately prior to the completion of the IPO, and upon issuance of the common shares, the contingent equity liability was reclassified to equity. The following table provides a roll forward of the aggregate fair values of the Company's warrant liability, derivative liability and contingent equity liability, for which fair value is determined by Level 3 inputs: Warrant Liability Derivative Liability Contingent Equity Liability December 31, 2016 780 512 18,938 Change in fair value 3,241 (512) 13,082 Issuance of common shares in settlement of contingent equity liability — — (32,020) December 31, 2017 4,021 — — Change in fair value 1,182 — — Reclassification to equity (5,203) — — Balance as at December 31, 2018 $ — $ — $ — Beneficial Conversion Feature In connection with the second tranche closing of Series A preferred shares on February 17, 2017, the Company determined that the conversion option associated with the shares sold met the definition of a beneficial conversion feature ("BCF") as the fair value of the underlying common shares exceeded the adjusted conversion price. The BCF was recognized at its fair value of $12,006 as a reduction to the carrying value of the Series A preferred shares and a corresponding adjustment to additional paid-in capital. The fair value was determined using Level 3 inputs, equal to the product of the number of shares sold in the second tranche closing multiplied by the difference between the adjusted conversion price and the per share value of common shares at the commitment date (see Note 10). In May 2017, upon the completion of the Company's IPO, all of the outstanding Series A preferred shares were automatically converted into an aggregate of 9,358,560 common shares. Upon conversion of the Series A preferred shares, the remaining unamortized BCF was reclassified to additional paid-in capital as a deemed dividend.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2018
Prepaid Expense and Other Assets, Current [Abstract]
Prepaid Expenses and Other Current Assets	Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following: December 31, 2018 2017 Prepaid clinical trial costs $ 7,210 $ 4,642 Prepaid insurance 393 455 Other 487 100 $ 8,090 $ 5,197

Equity Method Investment

Equity Method Investment	12 Months Ended
	Dec. 31, 2018
Equity Method Investments and Joint Ventures [Abstract]
Equity Method Investment	Equity Method Investment On August 29, 2016, the Company executed a stock purchase agreement with Kleo Pharmaceuticals, Inc. ("Kleo"), a privately held Delaware corporation, to purchase 3,000,000 shares of Kleo's common stock at an initial closing, with a commitment to purchase an aggregate of 5,500,000 additional shares of common stock, in each case at a share price of $1.00 per share (the "Kleo SPA"). Kleo is a development-stage biopharmaceutical company focused on advancing the field of immunotherapy by developing small molecules that emulate biologics. The Company purchased 3,000,000 shares upon the initial closing on August 31, 2016, and the remaining 5,500,000 shares were to be purchased in four equal tranches of 1,375,000 shares beginning six months from the initial closing and then every three months thereafter. In connection with the initial investment, the Company received the right to designate two of the members of Kleo's board of directors. The Company completed all four of remaining tranche purchases in March, June, October 2017 and January 2018, with each tranche purchase consisting of 1,375,000 shares for cash consideration of $1,375. In March 2017, the Company purchased 500,000 shares of Kleo common stock directly from a co-founder of Kleo for consideration of $250 in cash and 32,500 common shares of the Company. In addition to these purchases, in October 2017, the Company purchased an additional aggregate of 2,049,543 shares for cash consideration of $2,253 which allowed the Company to maintain its relative ownership interest in Kleo. As of December 31, 2017, the Company's ownership interest in the outstanding stock of Kleo was 43.3%. Upon completion of the fourth and final tranche investment in January 2018, the Company's ownership increased to 46.6%. In November 2018, the Company participated in Kleo's Series B funding raise. The Company purchased 1,420,818 shares for cash consideration of $5,000. As of the close of the Series B funding raise, and as of December 31, 2018, the Company's ownership interest in the outstanding common stock of Kleo was 41.9%. The Company has a variable interest in Kleo through its equity investment. Kleo is a variable interest entity due to the equity investment at risk being insufficient to finance its activities. An assessment of whether or not the Company has the power to direct activities that most significantly impact Kleo's economic performance and to identify the party that obtains the majority of the benefits of the investment was performed as of December 31, 2018 and 2017, and will be performed as of each subsequent reporting date. After each of these assessments, the Company concluded that the activities that most significantly impact Kleo's economic performance are the ability to direct the research activities, the ability to select vendors to perform the research, the ability to maintain research staff and the ability to raise additional funds, each of which are directed by Kleo. Based on the outcome of these assessments, the Company concluded that the investment should be accounted for under the equity method. The Company has recorded its investments in Kleo to date based on the costs of those investments, as adjusted for the Company's proportional share of Kleo's net income or loss in each period. The difference between the cost of the Company's investments in Kleo and its proportionate share of the net assets of Kleo was allocated to goodwill and indefinite-lived intangible assets. The Company records future adjustments to the carrying value of its investment at each reporting date equal to its proportionate share of Kleo's net loss for the corresponding period. The Company recorded other expense and a corresponding reduction in the carrying value of its investment in Kleo of $2,808 and $1,885 for its proportionate share of Kleo's net loss for the years ended December 31, 2018 and 2017, respectively. The carrying value of the Company's investment in Kleo was $11,414 and $7,847 as of December 31, 2018 and 2017, respectively, and is reported as equity method investment on the consolidated balance sheet. The carrying value of the investment represents the Company's maximum loss exposure as of December 31, 2018. The following table provides a roll forward of the carrying value of the Company's equity method investment: Carrying Value Balance as at December 31, 2016 $ 2,753 Purchase of Kleo common stock 6,979 Loss recognized in connection with equity method investment (1,885) Balance as at December 31, 2017 7,847 Purchases of Kleo common stock 6,375 Loss recognized in connection with equity method investment (2,808) Balance as at December 31, 2018 $ 11,414 Summarized financial information for Kleo is as follows: December 31, 2018 2017 Current assets $ 23,762 $ 8,388 Total assets $ 24,048 $ 8,746 Current liabilities $ 1,598 $ 1,415 Total liabilities $ 2,054 $ 4,201 Year Ended December 31, 2018 2017 2016 Revenue $ — $ — $ — Loss from operations $ (6,501) $ (5,646) $ (3,764) Net loss $ (6,334) $ (5,658) $ (3,727)

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	Property and Equipment, Net Property and equipment, net consisted of the following: December 31, 2018 2017 Building and Land $ 2,200 $ — Building Improvements 3,210 — Computer Hardware 420 163 Furniture & Fixtures 280 — Office Equipment 441 26 $ 6,551 $ 189 Accumulated depreciation (303) (43) Construction in progress — 2,198 $ 6,248 $ 2,344 In August 2017, the Company entered into a lease agreement to consolidate our headquarters into a free standing building in New Haven, Connecticut, which we began occupying during the fourth quarter of 2018. The Company had the option to purchase the property for $2,700 and executed that option in December 2018. Depreciation expense was $261, $35 and $5 for the years ended December 31, 2018, 2017 and 2016, respectively. Assets under the Company's financing lease included in construction in progress were $0 and $1,787 as of December 31, 2018 and December 31, 2017, respectively (see Note 16).

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued Expenses	Accrued Expenses Accrued expenses consisted of the following: December 31, 2018 2017 Accrued employee compensation and benefits $ 108 $ 89 Accrued clinical trial costs 6,753 3,582 Accrued professional fees 1,636 390 Lease liability — 362 Other 285 285 $ 8,782 $ 4,708

Liabilities Related to Sale of

Liabilities Related to Sale of Future Royalties	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Liabilities Related to Sale of Future Royalties	Liability Related to Sale of Future Royalties In June 2018, pursuant to the Funding Agreement entered into by the Company and RPI, a Delaware statutory trust, the Company issued to RPI the right to receive certain revenue participation payments, subject to certain reductions, based on the future global net sales of the Products for each calendar quarter during the royalty term contemplated by the Funding Agreement ("Revenue Participation Right"), in exchange for $100,000 in cash. Specifically, the participation rate commences at 2.1% on annual global net sales of up to and equal to $1,500,000, declining to 1.5% on annual global net sales exceeding $1,500,000. In addition, the Company had the option to repurchase 100% of the Revenue Participation Right from RPI for a purchase price of $155,000, if the Company entered into a definitive agreement to consummate a change of control. The Company did not exercise the Buy-Back Option which expired in July 2018. Concurrent with the Funding Agreement, the Company entered into a Purchase Agreement with RPI. Pursuant to the Purchase Agreement, the Company sold 1,111,111 common shares of the Company to RPI at a price of $45.00 per share, for gross proceeds of $50,000. The Company concluded that there were two units of accounting for the consideration received comprised of the liability related to sale of future royalties and the common shares. The Company allocated the $100,000 from the Funding Agreement and $50,000 from the Purchase Agreement among the two units of accounting on a relative fair value basis at the time of the transaction. The Company allocated $106,047 in transaction consideration to the liability, and $43,953 to the common shares. The Company determined the fair value of the common shares based on the closing stock price on the transaction date, adjusted for the trading restrictions. The transaction costs incurred related to the transactions with RPI of $377 were allocated in proportion to the allocation of total consideration to the two units of accounting. The effective interest rate under the Funding Agreement, including transaction costs, as of December 31, 2018 is approximately 22%. The following table shows the activity within the liability related to sale of future royalties for the twelve months ended December 31, 2018: Liability Related to Sale of Future Royalties Transaction date balance $ 106,047 Non-cash interest expense recognized, net of transaction cost amortization 11,726 Balance at December 31, 2018 117,773 Less: Unamortized transaction costs (258) Carrying value at December 31, 2018 $ 117,515

Warrants

Warrants	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Warrants	Warrants Credit Agreement On August 30, 2016, the Company entered into a one The Company recognized interest expense of $906 and $385 during the twelve months ended months ended December 31, 2017 and 2016, respectively. The Company recognized $784 and $347 related to the accretion of the debt discount during the twelve months ended December 31, 2017 and 2016, respectively. Notes Payable to Related Parties On December 31, 2016, the Company entered into stock purchase agreements with each of the stockholders of Biohaven Pharmaceuticals, Inc. ("BPI"), acquiring 100% of the issued and outstanding shares of BPI for aggregate purchase consideration of $595. The Company funded the acquisition through the issuance of promissory notes to each of the former stockholders of BPI. The former stockholders of BPI are shareholders of the Company and also serve as the Company's Chairman of the board of directors, Chief Executive Officer, and Chief Medical Officer, respectively. The notes were payable in five annual payments, the first four of which were interest only, with the final payment to include the principal balance outstanding plus any accrued and unpaid interest. The notes bore interest at a rate of 4.5% per annum and had a maturity date of December 31, 2021. The notes became immediately due and payable upon specified events, including immediately prior to the consummation of an initial public offering of the Company's common shares or upon the occurrence of a change of control of the Company. In connection with the closing of the Company's IPO in May 2017, the notes were paid in full. Guarantor and Co-Guarantor Warrants In connection with entering into the Credit Agreement, the Company issued warrants to purchase common shares to two of the Company’s directors in connection with a guarantee of its obligations under the agreement. The Company previously classified the warrants as a liability on its consolidated balance sheet because each warrant represented a freestanding financial instrument that was not indexed to the Company’s own shares. The warrant liability was initially recorded at fair value upon entering into the credit agreement and was subsequently remeasured to fair value at each reporting date. Changes in the fair value of the warrant liability was recognized as a component of other income (expense), net in the consolidated statement of operations and comprehensive loss. On January 26, 2018, the anti-dilution price protection provisions contained within the warrants issued to each of the guarantor and co-guarantor of the Credit Agreement expired. Both the guarantor and co-guarantor are members of the Company’s Board of Directors. Changes in the fair value of the warrant liability, until expiration of the anti-dilution price protection provisions, are recognized as a component of other income (expense), net, in the Company’s consolidated statement of operations and comprehensive loss. Upon expiration of the provision, the Company discontinued classification of these warrants as a liability, and has accordingly reclassified the fair value of $5,203 to additional paid-in capital within shareholders’ equity. The fair value of the warrant liability was $4,021 at December 31, 2017. The following table provides income (expense) related to the warrant liability which the Company records net within other income (expense) in the consolidated statements of operations: Twelve Months Ended December 31, 2018 2017 2016 Income (expense) from change in fair value of warrant liability $ (1,182) $ (3,241) $ 154

Convertible Preferred Shares

Convertible Preferred Shares	12 Months Ended
	Dec. 31, 2018
Temporary Equity Disclosure [Abstract]
Convertible Preferred Shares	Convertible Preferred Shares In October 2016, the Company issued and sold an aggregate of 4,305,209 Series A preferred shares (the “Series A First Closing”). The preferred share purchase agreement provided for the issuance of additional Series A preferred shares in a second and final tranche (the “Series A Second Closing”). Also, in October 2016, the Company issued 538,150 Series A preferred shares in satisfaction of the obligation to issue the first tranche of contingently issuable equity under the Company’s license agreement with AstraZeneca. In February 2017, the Company completed the Series A Second Closing through the issuance and sale of an aggregate of 4,305,182 Series A preferred shares. The conversion option associated with the Series A preferred shares sold in the second closing met the definition of a BCF as the fair value of the underlying common shares exceeded the stated conversion price. Upon the sale and issuance of the Series A preferred shares, $2,406 of the BCF was immediately accreted, as this represented the difference between the stated conversion price and per share value of the common shares. The remaining portion of the BCF was being amortized using the effective interest method over the period from the date of issuance to the date of the earliest possible conversion, October 1, 2017. In May 2017, upon the completion of the Company's IPO, all of the outstanding Series A preferred shares were automatically converted into an aggregate of 9,358,560 common shares. Upon conversion of the Series A preferred shares, the remaining unamortized BCF was reclassified to additional paid-in capital as a deemed dividend.

Common Shares

Common Shares	12 Months Ended
	Dec. 31, 2018
Stockholders' Equity Note [Abstract]
Common Shares	Common Shares As of December 31, 2016, the Company had authorized the Company to issue 38,000,000 no par value common shares. On April 21, 2017, the Company effected an increase in the number of authorized common shares to 50,000,000 shares. Additionally, in connection with the completion of its IPO in May 2017, the Company amended its memorandum and articles of association to authorize the issuance of up to 200,000,000 no par value common shares. Each common share entitles the holder to one vote on all matters submitted to a vote of the Company's shareholders. Common shareholders are entitled to receive dividends, as may be declared by the board of directors. In February 2016, the Company issued 429,000 common shares at an issuance price of $7.00 per share for proceeds of $2,980, net of issuance costs of $23. In May 2016 and July 2016, the Company issued an aggregate of 1,090,500 common shares at an issuance price of $7.70 per share for proceeds of $8,299, net of issuance costs of $97. In July 2016, concurrently with the issuance of the Company's common shares to Connecticut Innovations Incorporated ("CII"), the Company and CII entered into a put agreement (the "Put Agreement"). The Put Agreement grants CII the right to sell (the "Put Option") to the Company all or any part of CII's warrant rights (if any), shares (if any) or notes (if any). The Put Option becomes exercisable upon the Company's breach of the covenant to maintain a presence in Connecticut, as defined in the Put Agreement. The right to put the shares terminated on October 31, 2017, upon expiration of the lock-up period following the completion of the Company's IPO. On May 3, 2017, the Company's registration statement on Form S-1 relating to the IPO was declared effective by the SEC. The IPO closed on May 9, 2017 and the Company issued and sold 9,900,000 common shares at a public offering price of $17.00 per share for net proceeds of $152,651 after deducting underwriting discounts and commissions of $11,781 and other offering expenses of $3,868. Upon the closing of the IPO, all convertible preferred shares then outstanding converted into an aggregate of 9,358,560 common shares. In addition, on May 9, 2017, the underwriters of the IPO fully exercised their option to purchase additional shares, and on May 11, 2017, the Company issued and sold 1,485,000 common shares for net proceeds of $23,478 after deducting underwriting discounts and commissions of $1,767. The aggregate net proceeds to the Company from the IPO, after deducting underwriting discounts and commissions and offering expenses, were $176,128. In connection with the completion of its IPO, the Company issued an aggregate of 1,883,523 common shares to BMS and AstraZeneca in satisfaction of obligations to contingently issue equity securities pursuant to the license agreements (see Note 13), for no additional consideration. In December 2018, we closed on an underwritten public offering of 3,859,060 of common shares, including the full exercise of the underwriters' option to purchase additional shares, at a price to the public of $37.25 per share. The aggregate gross proceeds to the Company from the offering, before deducting the underwriting discounts and commissions and offering expenses payable, were approximately $143,750. Private Placements In March 2018, the Company sold an aggregate of 2,000,000 common shares in a private placement at a price of $27.50 per share, for net proceeds of $52,000 (“Private Placement”) after deducting underwriting discounts and commissions of $2,800 and other offering expenses of $200. Subsequent to the closing of the Private Placement, the Company paid BMS the $50,000 upfront payment under the BMS Amendment (see Note 13). In June 2018, pursuant to the Purchase Agreement between the Company and RPI (Note 8), the Company sold 1,111,111 common shares to RPI at a price of $45.00 per common share for net proceeds of $49,889 after deducting offering expenses of $111. 2018 License Agreement with AstraZeneca In September 2018, the Company entered into a License Agreement (the “2018 AstraZeneca Agreement”) with AstraZeneca AB (“AstraZeneca”). Under the 2018 AstraZeneca Agreement, the Company paid AstraZeneca an upfront cash payment of $3,000 and 109,523 shares valued at $4,080 on the settlement date (see Note 13). Agreement with ALS Biopharma, LLC and Fox Chase Chemical Diversity Center Inc. In January 2018, ALS Biopharma exercised a warrant for the purchase of 275,000 shares through a net share settlement, resulting in an issuance of 228,219 shares. In April 2018, ALS Biopharma exercised a warrant for the purchase of 325,000 shares through a net share settlement, resulting in an issuance of 261,140 shares.

Share-Based Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-Based Compensation	Share-Based Compensation 2014 Equity Incentive Plan The Company's 2014 Equity Incentive Plan, as amended (the "2014 Plan"), provided for the Company to sell or issue common shares or restricted common shares, or to grant incentive stock options or nonqualified stock options for the purchase of common shares, to employees, members of the board of directors and consultants of the Company. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or their committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the common share on the date of grant and the term of stock option may not be greater than 10 years. The total number of common shares that may be issued under the 2014 Plan was 4,000,000 shares as of December 31, 2016. In January 2017, the Company effected an increase, effective October 28, 2016, in the number of common shares reserved for issuance under the 2014 Plan from 4,000,000 to 4,899,230 shares. Upon effectiveness of the 2017 Plan, there are no further shares authorized for grant under the 2014 Plan. 2017 Equity Incentive Plan In April 2017, the Company's shareholders approved the 2017 Equity Incentive Plan (the "2017 Plan"), which became effective on May 3, 2017 in connection with the Company's IPO. The 2017 Plan provides for the grant of incentive share options, nonstatutory share options, share appreciation rights, restricted share awards, restricted share unit awards, performance-based share awards and other share-based awards. Additionally, the 2017 Plan provides for the grant of performance cash awards. Upon the effectiveness of the 2017 Plan, there were 2,713,113 shares authorized for issuance under the 2017 Plan. As of December 31, 2017, 1,138,903 shares remained available for future grant under the 2017 Plan. In January 2018, the Board of Directors approved an additional 1,437,227 shares to be issued under the 2017 Plan. As of December 31, 2018, 681,307 shares remained available for future grant under the 2017 Plan. In January 2019, the Board of Directors approved an additional 1,767,901 shares to be issued under the 2017 Plan. Vesting periods are determined at the discretion of the board of directors. Stock options typically vest over three or four years. The maximum contractual term is 10 years. During the years ended December 31, 2018, 2017 and 2016 the Company granted options to purchase 1,810,000, 2,335,106 and 417,875 common shares to employees and directors, respectively. The Company recorded non-cash share-based compensation expense for options granted to employees and directors of $11,246, $5,210 and $2,284 during the years ended December 31, 2018, 2017 and 2016, respectively. During the year ended December 31, 2018, 2017 and 2016 the Company granted options to purchase 145,000, 273,537 and 199,050 common shares to non-employees, respectively. The Company recorded non-cash share-based compensation expense for options granted to non-employees of $5,679, $8,029 and $2,319 during the years ended December 31, 2018, 2017 and 2016, respectively. The Company measures and records the value of non-employee options over the period of time services are provided and, as such, unvested portions are subject to remeasurement at subsequent reporting periods. Stock Option Valuation The assumptions that the Company used to determine the grant-date fair value of stock options granted to employees and directors under the 2014 Plan and the 2017 Plan (collectively, the "Plans") were as follows, presented on a weighted average basis: Year Ended December 31, 2018 2017 2016 Risk-free interest rate 2.91 % 2.10 % 2.19 % Expected term (in years) 6.25 6.02 5.75 Expected volatility 73.03 % 73.26 % 70.58 % Expected dividend yield — % — % — % Exercise price $ 32.35 $ 18.47 $ 9.29 The assumptions that the Company used to determine the grant-date fair value of stock options granted to non-employees under the Plans were as follows, presented on a weighted average basis: Year Ended December 31, 2018 2017 2016 Risk-free interest rate 3.06 % 2.35 % 2.54 % Expected term (in years) 10 10 10 Expected volatility 74.50 % 71.12 % 67.16 % Expected dividend yield — % — % — % Exercise price $ 32.42 $ 18.23 $ 9.29 Stock Options Stock option activity under the Plans is summarized as follows: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2017 6,151,643 $ 9.97 8.42 $ 107,072 Granted 1,955,000 $ 32.35 9.70 Exercised (577,288) $ 6.26 Forfeited (85,176) $ 22.54 Outstanding as of December 31, 2018 7,444,179 $ 15.99 8.05 $ 156,518 Options exercisable as of December 31, 2018 3,854,598 $ 6.26 6.92 $ 118,452 Options unvested as of December 31, 2018 3,589,581 $ 26.46 9.26 $ 38,066 The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company's common shares for those stock options that had exercise prices lower than the fair value of the Company's common shares. The total intrinsic value of stock options for the years ended December 31, 2018, 2017 and 2016 was $156,518, $107,072 and $15,991, respectively. The weighted average grant-date fair value per share of stock options granted for the years ended December 31, 2018, 2017 and 2016 was $22.00, $12.31 and $4.09, respectively. The total fair value of options vested for the years ended December 31, 2018, 2017 and 2016 was $25,876, $15,494 and $3,381, respectively. Non-Cash Share-Based Compensation Non-cash share-based compensation expense was classified in the consolidated statements of operations and comprehensive loss as follows: Year Ended December 31, 2018 2017 2016 Research and development expenses $ 8,371 $ 6,933 $ 2,382 General and administrative expenses 8,554 6,306 2,221 $ 16,925 $ 13,239 $ 4,603 As of December 31, 2018, total unrecognized compensation cost related to the unvested share-based awards was $61,583, which is expected to be recognized over a weighted average period 3.30 years.

License and Other Agreements

License and Other Agreements	12 Months Ended
	Dec. 31, 2018
License and Other Agreements
License and Other Agreements	License and Other Agreements Yale Agreement In September 2013, the Company entered into an exclusive license agreement with Yale (the "Yale Agreement") 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, related to the use of riluzole in treating various neurological conditions, such as general anxiety disorder, post-traumatic stress disorder and depression. As part of the consideration for this license, the Company issued Yale 250,000 common shares and granted Yale the right to purchase up to 10% of the securities issued in specified future equity offerings by the Company, in addition to the obligation to issue shares to prevent anti-dilution. The obligation to contingently issue equity to Yale, which was no longer outstanding as of December 31, 2018, was determined to be a liability, which was accounted for at its fair value of zero at each reporting date. The Yale Agreement provided for a change-of-control payment to Yale upon the occurrence of a change-of-control event, as defined in the agreement, including an IPO. Upon the occurrence of a change-of-control event, the Company was obligated to pay to Yale the lesser of (i) 5% of the dollar value of all initial and future potential consideration paid or payable by the acquirer or (ii) $1,500. If the change-of-control event was an IPO, the amount the Company would have been obligated to pay to Yale would have been reduced by the value of Yale's equity investment in the Company on the first day that Yale was free to sell its equity interest. The Company classified the change-of-control payment obligation as a liability on its consolidated balance sheet because it represented a contingent obligation to pay a variable amount of cash that may be based, in part, on the value of the Company's own shares. The issuance date fair value of the derivative liability of $14 was recognized as a research and development expense upon entering the agreement with Yale. The Company continued to remeasure the derivative liability to fair value at each reporting date and recognized changes in the fair value of the derivative liability through October 31, 2017. The derivative liability upon expiration of the lock-up period was determined to be $0 based on the value of the Company's shares on that date. The Company recorded other income (expense) of $0, $512 and $(65) during the years ended December 31, 2018, 2017 and 2016, respectively, for the change in the fair value of the derivative liability. The fair value of the derivative liability was $0 and $0 as of December 31, 2018 and 2017, respectively (see Note 3). In addition, the Company agreed to pay Yale up to $2,000 upon the achievement of specified regulatory milestones and annual royalty payments of a low single-digit percentage based on net sales of products from the licensed patents, subject to a minimum amount of up to $1,000 per year. If the Company grants any sublicense rights under the Yale Agreement, it must pay Yale a low single-digit percentage of sublicense income that it receives. To date, no milestone or royalty payments have been made under this agreement. The Yale Agreement also requires the Company to meet certain due diligence requirements based upon specified milestones. The Company can elect to extend the deadline for its compliance with the due diligence requirements by a maximum of one year upon the payment to Yale of up to $150. The Company is also required to reimburse Yale for any fees that Yale incurs related to the filing, prosecution, defending and maintenance of patent rights licensed under the Yale Agreement. In the event that the Company fails to make any payments, commits a material breach, fails to maintain adequate insurance or challenges the patent rights of Yale, Yale can terminate the Yale Agreement. The Company can terminate the Yale Agreement (i) upon 90 days' notice to Yale, (ii) if Yale commits a material breach of the Yale Agreement or (iii) as to a specific country if there are no valid patent rights in such country. The Yale Agreement expires on a country-by-country basis upon the later of the date on which the last patent rights expire in such country or ten years from the date of the first sale of a product incorporating the licensed patents. MGH Agreement In September 2014, the Company entered into a license agreement (the "MGH Agreement") with The General Hospital Corporation d/b/a Massachusetts General Hospital ("MGH"), pursuant to which MGH granted the Company a license to certain patent rights for the commercial development, manufacture, distribution and use of any products or processes resulting from development of those patent rights, related to treating depression with a combination of ketamine and scopolamine. The Company is also obligated to pay MGH annual license maintenance fees and future milestone payments of up to $750 upon the achievement of specified clinical and regulatory milestones and up to $2,500 upon the achievement of specified commercial milestones. The Company has also agreed to pay MGH royalties of a low single-digit percentage based on net sales of products licensed under the agreement. If the Company receives revenue from sublicensing any of its rights under the agreement, the Company is also obligated to pay a portion of that revenue to MGH. To date, no milestone payments have been made under this agreement. The MGH Agreement also requires the Company to meet certain due diligence requirements based upon specified milestones. The Company can elect to extend the deadline for its compliance with the due diligence requirements by a maximum of one year by making payments to MGH of up to $300 in the aggregate. The Company is required to reimburse MGH for any fees that MGH incurs related to the filing, prosecution, defending, and maintenance of patent rights licensed under the agreement. The MGH Agreement expires upon expiration of the patent rights under the MGH Agreement, unless earlier terminated by either party. ALS Biopharma Agreement In August 2015, the Company entered into an agreement (the "ALS Biopharma Agreement") with ALS Biopharma and Fox Chase Chemical Diversity Center Inc. ("FCCDC"), pursuant to which ALS Biopharma and FCCDC assigned the Company their worldwide patent rights to a family of over 300 prodrugs of glutamate modulating agents, including troriluzole, as well as other innovative technologies. Under the ALS Biopharma Agreement, the Company is obligated to use commercially reasonable efforts to commercialize and develop markets for the patent products. The Company is obligated to pay $3,000 upon the achievement of specified regulatory milestones with respect to the first licensed product and $1,000 upon the achievement of specified regulatory milestones with respect to subsequently developed products, as well as royalty payments of a low single-digit percentage based on net sales of products licensed under the agreement, payable on a quarterly basis. To date, no milestone or royalty payments have been made under this agreement. In connection with the ALS Biopharma Agreement, the Company also issued to ALS Biopharma (i) 50,000 common shares; (ii) an immediately exercisable warrant to purchase 275,000 common shares at an exercise price of $5.60 per share; and (iii) a warrant to purchase 325,000 common shares at an exercise price of $5.60 per share, which warrant would become exercisable upon the Company's achievement of a specified regulatory milestone which was met in May 2016 (see Note 9). The ALS Biopharma Agreement terminates on a country-by-country basis as the last patent rights expire in each such country. If the Company abandons its development, research, licensing or sale of all products covered by one or more claims of any patent or patent application assigned under the ALS Biopharma Agreement, or if the Company ceases operations, it has agreed to reassign the applicable patent rights back to ALS Biopharma. The Company recorded research and development expenses of $0, $0 and $3,127 during the years ended December 31, 2018, 2017 and 2016, respectively, as a result of the ALS Biopharma Agreement, which amounts consist of the fair value of the shares and warrants upon their issuance to ALS Biopharma. Rutgers Agreement In June 2016, the Company entered into an exclusive license agreement (the "Rutgers Agreement") with Rutgers, The State University of New Jersey ("Rutgers"), licensing several patents and patent applications related to the use of riluzole to treat various cancers. Under the Rutgers Agreement, the Company is required to pay Rutgers annual license maintenance fees until the first commercial sale of a licensed product, at which point the Company will pay Rutgers minimum annual royalties. The Company is also obligated to pay Rutgers up to $825 in the aggregate upon the achievement of specified clinical and regulatory milestones. The Company agreed to pay Rutgers royalties of a low single-digit percentage of net sales of licensed products sold by the Company, its affiliates or its sublicensees, subject to a minimum amount of up to $100 per year. If the Company grants any sublicense rights under the Rutgers Agreement, the Company must pay Rutgers a low double-digit percentage of sublicense income it receives. Under the Rutgers Agreement, in the event that the Company experiences a change of control or sale of substantially all of its assets prior to the initiation of a Phase 3 clinical trial related to products licensed under the agreement, and such change of control or sale results in a full liquidation of the Company, the Company will be obligated to pay Rutgers a change-of-control fee equal to 0.30% of the total value of the transaction, but not less than $100. The Company determined that the change-of-control payment should be accounted for as a liability. The fair value of the obligation for all periods presented was $0 based on the Company's assessment that the probability of a change-in-control event occurring prior to the initiation of a Phase 3 clinical trial related to products licensed under the agreement was remote. The Rutgers Agreement also requires the Company to meet certain due diligence requirements based upon specified milestones. The Company can elect to extend the deadline for its compliance with the due diligence requirements by a maximum of one year upon payments to Rutgers of up to $500 in the aggregate. Under the Rutgers Agreement, the Company is required to reimburse Rutgers for any fees that Rutgers incurs related to the filing, prosecution, defending, and maintenance of patent rights licensed under the agreement. The Rutgers Agreement expires upon expiration of the patent rights under the agreement or ten years from the date of first commercial sale of a licensed product, whichever is later, unless terminated by either party. BMS Agreement In July 2016, the Company entered into an exclusive, worldwide license agreement with BMS (the "BMS Agreement") for the development and commercialization rights to rimegepant and BHV-3500, as well as other CGRP-related intellectual property. In exchange for these rights, the Company agreed to pay BMS initial payments, milestone payments and royalties on net sales of licensed products under the agreement. The Company is obligated to make milestone payments to BMS upon the achievement of specified development and commercialization milestones. The development milestone payments due under the agreement depend on the licensed product being developed. With respect to rimegepant, the Company is obligated to pay up to $127,500 in the aggregate upon the achievement of the development milestones. For any product other than rimegepant, the Company is obligated to pay up to $74,500 in the aggregate upon the achievement of the development milestones. In addition, the Company is obligated to pay up to $150,000 for each licensed product upon the achievement of commercial milestones. If the Company receives revenue from sublicensing any of its rights under the agreement, it is also obligated to pay a portion of that revenue to BMS. The Company is also obligated to make tiered royalty payments to BMS based on annual worldwide net sales, with percentages in the low to mid-teens. Under the BMS Agreement, the Company is obligated to use commercially reasonable efforts to develop licensed products and to commercialize at least one licensed product using the patent rights licensed from BMS and is solely responsible for all development, regulatory and commercial activities and costs. The Company is also required to reimburse BMS for any fees that BMS incurs related to the filing, prosecution, defending, and maintenance of patent rights licensed under the BMS Agreement. Under the BMS Agreement, BMS transferred to the Company manufactured licensed products, including certain materials that will be used by the Company to conduct clinical trials. The BMS Agreement will terminate on a licensed product-by-licensed product and country-by-country basis upon the expiration of the royalty term with respect to each licensed product in each country. BMS has the right to terminate the agreement upon the Company's insolvency or bankruptcy, the Company's uncured material breach of the agreement, including the failure to meet its development and commercialization obligations, or if the Company challenges any of BMS's patent rights. The Company has the right to terminate the BMS Agreement if BMS materially breaches the agreement or if, after the Company provides notice, it chooses not to move forward with development and commercialization in a specific country. The BMS Agreement required the Company to complete a financing transaction with gross proceeds of at least $30,000, of which a minimum of $22,000 was to be from investment in equity prior to October 17, 2016, unless extended by mutual agreement of the Company and BMS. The BMS Agreement was amended, effective October 14, 2016, to extend the deadline for completing the financing transaction to October 31, 2016, on which date the Series A First Closing was completed (see Note 13). Under the BMS Agreement, the Company also agreed to issue BMS common shares in the amount of $12,500, which shares are contingently issuable upon the earliest to occur of (i) the initiation of a Phase 3 trial for the first licensed compound to reach such milestone, (ii) the Company's IPO or (iii) an event resulting in the change of control of the Company. Under the terms of the BMS Agreement, if the qualifying financing transaction involves the issuance of preferred shares, BMS is entitled to receive preferred shares instead of common shares, at its option. BMS also had the right to purchase up to 8%, on a fully diluted basis, of shares issued in a qualifying financing transaction (as defined in the BMS Agreement) on the same terms and rights as all other investors involved in the financing. The number of shares issuable to BMS under the agreement will be determined by dividing $12,500 by a price per share equal to the lower of (i) the price per share paid by investors in the Series A First Closing, or $9.2911 (see Note 13), or (ii) the price per share paid by investors in any subsequent financing event that occurs prior to the events specified above. The obligation to contingently issue equity to BMS is classified as a liability on the consolidated balance sheet because it represents an obligation to issue a variable number of shares for a fixed dollar amount. Upon entering into the BMS Agreement, the issuance-date fair value of the contingent equity liability of $13,125 was recognized as research and development expense in the consolidated statement of operations and comprehensive loss. The Company remeasured the fair value of the contingent equity liability at each reporting date since the date of issuance, recognizing changes in the fair value of the contingent equity liability as a component of other income (expense), net in the consolidated statement of operations and comprehensive loss. Changes in the fair value of the contingent equity liability continued to be recognized until the occurrence of a triggering event, which occurred in May 2017 with the completion of the IPO. In May 2017, in connection with the completion of its IPO, the Company issued 1,345,374 common shares to BMS in satisfaction of its obligation to contingently issue equity securities pursuant to the license agreement and remeasured the contingent equity liability to fair value. The Company recognized expense of $0, $8,809 and $13,125 during the years ended December 31, 2018, 2017 and 2016, respectively, as a result of changes to the fair value of the contingent equity liability prior to its extinguishment in May 2017. The Company recorded $2,000 and $5,000 of research and development expense related to the BMS Agreement during the year ended December 31, 2018 and 2017, respectively, for the achievement of specified milestones. Amendment to License Agreement with BMS In March 2018, the Company entered into an Amendment to License Agreement with BMS (the “BMS Amendment”), which amends the License Agreement between the Company and BMS from July 2016 (the “Original License Agreement” and, as amended by the BMS Amendment, the “BMS License Agreement”). Under the BMS Amendment, the Company paid BMS an upfront payment of $50,000 in return for a low single-digit reduction in the royalties payable on net sales of rimegepant and a mid single-digit reduction in the royalties payable on net sales of BHV-3500, recorded in Research and Development expense in the Consolidated Statements of Operations and Comprehensive Loss. Under the Original License Agreement, the Company was obligated to make tiered royalty payments based on annual worldwide net sales of licensed products upon their approval and commercialization, with percentages in the low- to mid-teens. The BMS Amendment also removes BMS’s right of first negotiation to regain its intellectual property rights or enter into a license agreement with the Company following the Company’s receipt of topline data from its Phase 3 clinical trials with rimegepant, and clarifies that antibodies targeting CGRP are not prohibited as competitive compounds under the non-competition clause of the Original License Agreement. The BMS License Agreement continues to provide the Company with exclusive global development and commercialization rights to rimegepant, BHV-3500 and related CGRP molecules, as well as related know-how and intellectual property. The Company’s obligations to make development and commercial milestone payments to BMS under the Original License Agreement remain unchanged. 2016 AstraZeneca Agreement In October 2016, the Company entered into an exclusive license agreement (the "2016 AstraZeneca Agreement") with AstraZeneca, pursuant to which AstraZeneca granted the Company a license to certain patent rights for the commercial development, manufacture, distribution and use of any products or processes resulting from development of those patent rights, including BHV-5000 and BHV-5500. In exchange for these rights, the Company agreed to pay AstraZeneca an upfront payment, milestone payments and royalties on net sales of licensed products under the agreement. The regulatory milestones due under the agreement depend on the indication of the licensed product being developed as well as the territory where regulatory approval is obtained. Development milestones due under the agreement with respect to Rett syndrome total up to $30,000, and, for any indication other than Rett syndrome, total up to $60,000. Commercial milestones are based on net sales of all products licensed under the agreement and total up to $120,000. The Company has also agreed to pay tiered royalties based on net sales of all products licensed under the agreement of mid-single-digit to low double-digit percentages. If the Company receives revenue from sublicensing any of its rights under the 2016 AstraZeneca Agreement, the Company is also obligated to pay a portion of that revenue to AstraZeneca. To date, no payments have been made related to these milestones or royalties. The Company is also required to reimburse AstraZeneca for any fees that AstraZeneca incurs related to the filing, prosecution, defending, and maintenance of patent rights licensed under the 2016 AstraZeneca Agreement. The 2016 AstraZeneca Agreement expires upon the expiration of the patent rights under the agreement, unless earlier terminated by either party, or on a country-by-country basis ten years after the first commercial sale. As part of the consideration under the 2016 AstraZeneca Agreement, the Company agreed to issue to AstraZeneca common shares in the amount of $10,000 if the Company completed a qualifying equity financing resulting in proceeds of at least $30,000 prior to December 29, 2016. Under the terms of the 2016 AstraZeneca Agreement, if the qualifying financing transaction involved the issuance of preferred shares, AstraZeneca would be entitled to receive preferred shares instead of common shares, at its option. The number of shares issued would be determined based on the price per share paid by investors in the qualifying financing transaction. Upon the occurrence of the qualifying financing transaction, 50% of the shares would be issuable upon the closing of the transaction (the "First Tranche") and the other 50% would become issuable upon the earlier of (i) the initiation of a Phase 2b or equivalent clinical trial of a product candidate based on the licensed patent rights or (ii) any liquidity event, including an IPO of the Company, any change of control of the Company or any assignment of the Company's rights and obligations under the 2016 AstraZeneca Agreement (the "Second Tranche"). The number of shares issuable to AstraZeneca in each of the First Tranche and the Second Tranche is determined by dividing $5,000 by the price per share paid by investors in the Company's Series A First Closing, or $9.2911 (see Note 11). In addition, AstraZeneca had the right to purchase up to 8%, on a fully diluted basis, of shares issued in such qualifying financing transaction, on the same terms and rights as all other investors involved in the financing. In October 2016, upon completion of the Series A First Closing (see Note 11), the contingency associated with the First Tranche of contingently issuable equity related to the occurrence of a qualified financing was satisfied. As a result, the Company issued to AstraZeneca 538,150 Series A preferred shares with an aggregate fair value of $5,000, or $9.2911 per share. Upon issuance of the 538,150 Series A preferred shares to AstraZeneca, the Company reclassified the contingent equity liability associated with the First Tranche of $5,000 to the carrying value of Series A preferred shares. The Company determined that the fair value of the contingent equity liability associated with the Second Tranche at each reporting date since the date of issuance, recognizing changes in the fair value of the contingent equity liability as a component of other income (expense), net in the consolidated statement of operations and comprehensive loss. Changes in the fair value of the contingent equity liability continued to be recognized until the occurrence of a triggering event, which occurred in May 2017 with the completion of the IPO. In May 2017, in connection with the completion of its IPO, the Company issued 538,149 common shares to AstraZeneca in satisfaction of its obligation to contingently issue the Second Tranche of equity securities pursuant to the license agreement and remeasured the contingent equity liability to fair value. The Company recognized expense of $4,273 and $938 during the years ended December 31, 2017 and 2016, respectively, as a result of changes to the fair value of the contingent equity liability prior to its extinguishment in May 2017. RPharm Agreement In November 2017, the Company entered into a consulting agreement with R PHARM US related to the commercial preparation for BHV-0223. In addition to fixed quarterly consulting expenses under the agreement, the Company agreed to pay up to $2,500 upon achievement of specific commercial milestones. During the year ended December 31, 2018 the Company paid $1,400 to R PHARM US under this agreement. Catalent Agreements for Rimegepant In January 2018, the Company entered into an exclusive world-wide license and development agreement with Catalent, Inc. pursuant to which the Company obtained certain license rights to the Zydis ODT technology for use with rimegepant. If the Company obtains regulatory approval or launches a rimegepant product that utilizes the Zydis ODT technology, the Company is obligated to pay Catalent up to $1,500 upon the achievement of specified regulatory and commercial milestones. If the Company commercializes a rimegepant product that utilizes the Zydis ODT technology, the agreement permits the Company to purchase the commercial product from Catalent at a fixed price, inclusive of a royalty. Under the agreement, Catalent will not develop or manufacture a formulation of any oral CGRP compound using Zydis ODT technology for itself or a third party until 2031, subject to certain minimum commercial revenues. Under this agreement, the Company is responsible for conducting clinical trials and preparing and filing regulatory submissions. The Company has the right to sublicense its rights under the agreement subject to Catalent’s prior written consent. Catalent has the right to enforce the patents covering the Zydis technology and to defend any allegation that a formulation using Zydis technology, such as rimegepant, infringes a third party’s patent. This agreement terminates on a country-by-country basis upon the later of (i) 10 years after the launch of the most recently launched product in such country and (ii) the expiration of the last valid claim covering each product in such country, unless earlier voluntarily terminated by the Company. This agreement automatically extends for one In July 2018, the Company entered into a commercial supply agreement with Catalent pursuant to which Catalent will exclusively manufacture and supply the Company's worldwide requirements for rimegepant in the Zydis ODT delivery formulation, if the Company pursues and receives regulatory approval of this formulation of rimegepant, for an initial term of five years after its commercial launch with optional two-year renewal periods. Under the agreement, Catalent will supply the rimegepant Zydis ODT product at a fixed price, inclusive of a royalty, and will not develop or manufacture a formulation of any oral CGRP compound using Zydis ODT technology for itself or a third party for a specified period of time, subject to certain minimum commercial revenues. Revenue Participation Right with RPI Finance Trust In June 2018, pursuant to the Funding Agreement entered into by the Company and RPI (Note 8), the Company granted to RPI the right to receive certain revenue participation payments, subject to certain reductions, based on the future global net sales of the Products, for each calendar quarter during the royalty term contemplated by the Funding Agreement, in exchange for $100.0 million in cash. Specifically, the participation rate commences at 2.1 percent on annual global net sales of up to and equal to $1.5 billion, declining to 1.5 percent on annual global net sales exceeding $1.5 billion. In addition, the Company had the option to repurchase 100% of the Revenue Participation Right from RPI for a purchase price of $155.0 million, if the Company entered into a definitive agreement to consummate a change of control. The Company did not exercise the Buy-Back Option which expired in July 2018. 2018 License Agreement with AstraZeneca In September 2018, the Company entered into the 2018 AstraZeneca Agreement. Under the 2018 AstraZeneca Agreement, the Company paid AstraZeneca an upfront cash payment of $3,000 and 109,523 shares valued at $4,080 on the date of settlement, both of which are included in research and development expense, and is obligated to pay milestone payments to AstraZeneca totaling up to $55,000 upon the achievement of specified regulatory and commercial milestones and up to $50,000 upon the achievement of specified sales-based milestones. In addition, we will pay AstraZeneca tiered royalties ranging from high single-digit to low double-digits based on net sales of specified approved products, subject to specified reductions. AstraZeneca granted Biohaven exclusive worldwide rights to develop and commercialize AZD3241, an oral myeloperoxidase (“MPO”) inhibitor that AstraZeneca progressed through Phase 2 clinical trials. We plan to conduct a Phase 3 clinical trial of this product candidate, which will now be referred to as verdiperstat, for the treatment of multiple system atrophy (“MSA”), a rare, rapidly progressive and fatal neurodegenerative disease with no cure or effective treatments. We are now solely responsible, and has agreed to use commercially reasonable efforts, for all development, regulatory and commercial activities related to verdiperstat. We may sublicense its rights under the Agreement and, if it does so, will be obligated to pay a portion of any milestone payments received from the sublicensee to AstraZeneca in addition to any milestone payments we would otherwise be obligated to pay. We are also now responsible for the prosecution and maintenance of the patents related to verdiperstat and has the first right to prosecute infringement of the patents and defend challenges to the validity or enforceability of the patents. The Agreement terminates on a country-by-country basis and product-by-product basis upon the expiration of the royalty term for such product in such country. Each royalty term begins on the date of the first commercial sale of the licensed product in the applicable country and ends on the later of 10 years from such first commercial sale or the expiration of the last to expire of the applicable patents in that country. The Agreement may be terminated earlier in specified situations, including termination for uncured material breach of the Agreement by either party, termination by AstraZeneca in specified circumstances, termination by us on a country-by-country basis with advance notice and termination upon a party’s insolvency or bankruptcy. License Agreement with the University of Connecticut In October 2018, the Company announced it had signed an exclusive, worldwide option and license agreement (the "UConn Agreement") with the University of Connecticut ("UConn") for the development and commercialization rights to UC1MT, a therapeutic antibody targeting extracellular metallothionein. Under this agreement, we have the option to acquire an exclusive, worldwide license to UC1MT and its underlying patents to develop and commercialize throughout the world in all human indications. If the Company chooses to exercise the option, it would be obligated to pay UConn upon the achievement of specified regulatory and commercial milestones, and royalties of a low single-digit percentage of net sales of licensed products sold by the Company, its affiliates or its sublicensees.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes As a company incorporated in the British Virgin Islands ("BVI"), the Company is principally subject to taxation in the BVI. Under the current laws of the BVI, tax on a company's income is assessed at a zero percent tax rate. As a result, the Company has not recorded any income tax benefits from its losses incurred in the BVI during each reporting period, and no net operating loss carryforwards will be available to the Company for those losses. BVI has historically outsourced all of the research and clinical development for its programs under a master services agreement with BPI (see Note 18). As a result of providing services under this agreement, BPI was profitable during the years ended December 31, 2018 and 2017, and BPI is subject to taxation in the United States. The Company's tax provision includes the effects of consolidating the results of operations of BPI, either as a variable interest entity for periods through the acquisition of BPI (see Note 18) or as of December 31, 2018 and 2017 as the Company's wholly owned subsidiary. Due to BPI's history of cumulative losses through September 30, 2016, the Company had recorded no tax benefits for the losses incurred by BPI through that date and had recorded a full valuation allowance against BPI's deferred tax assets, which consisted primarily of its U.S. net operating loss carryforwards for all periods through September 30, 2016. As of December 31, 2016, the Company fully utilized BPI's remaining U.S. net operating loss carryforwards and recorded a full release of the valuation allowance, which did not result in a material impact to the Company's income tax provision. As of December 31, 2018, we evaluated our deferred tax assets and determined that a full valuation allowance on these assets was appropriate due to the generation of tax credits in excess of forecasted taxes. The Company recorded an income tax provision during the year ended December 31, 2018 of $467 which primarily represents certain state taxes for the period and federal taxes due to general business credit limitations. Income (loss) before provision for income taxes consisted of the following: Year Ended December 31, 2018 2017 2016 BVI $ (246,829) $ (130,359) $ (63,677) Foreign (U.S.) 6,374 4,175 233 Loss before provision for income taxes $ (240,455) $ (126,184) $ (63,444) The provision for income taxes consisted of the following: Year Ended December 31, 2018 2017 2016 Current income tax provision: BVI $ — $ — $ — Foreign (U.S. federal and state) 467 997 99 Total current income tax provision 467 997 99 Deferred income tax provision (benefit): BVI — — — Foreign (U.S. federal and state) — 9 (9) Total deferred income tax provision (benefit) — 9 (9) Total provision for income taxes $ 467 $ 1,006 $ 90 A reconciliation of the BVI statutory income tax rate of 0% to the Company's effective income tax rate is as follows: Year Ended December 31, 2018 2017 2016 BVI statutory income tax rate .0 % .0 % .0 % Foreign tax rate differential .6 1.2 .1 Tax Credits (3.5) (2.7) .0 Change in valuation allowance 3.4 2.2 .0 Other (.3) .1 .0 Effective income tax rate .2 % .8 % .1 % Net deferred tax assets (liabilities) consisted of the following: December 31, 2018 2017 Deferred tax assets: Foreign net operating loss carryforwards $ — $ — Tax credits 11,396 2,790 Other 1 1 Total deferred tax assets 11,397 2,791 Deferred tax liabilities: Other (440) (7) Total deferred tax liabilities (440) (7) Valuation allowance (10,957) (2,784) Net deferred tax assets $ — $ — As of December 31, 2018 and 2017, the Company had no remaining foreign net operating loss carryforwards. The Company had federal and state research and development credits of $7,569 and $1,623 which begin to expire in 2037. As of December 31, 2018 the Company had federal orphan drug credits of $2,544 which can carryforward into 2038. On December 22, 2017, the Tax Cuts and Jobs Act ("The Act"), was signed into law, resulting in significant changes to the Internal Revenue Code of 1986, as amended. These changes include a federal statutory rate reduction from 35% to 21%, limitation on the amount of research and development expenses deductible per year beginning in years after 2021, reduction of the Orphan Drug Credit from 50% to 25% of qualified clinical testing expenditures, increased limitations on certain executive compensation, elimination of the Corporate Alternative Minimum Tax, and modifying or repealing other business deductions and credits. The revaluation of our deferred tax assets due to The Act was not material. Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2018 and 2017 were due primarily to generation of excess credits and were as follows: Year Ended December 31, 2018 2017 2016 Valuation allowance as of beginning of year $ 2,784 $ — $ 16 Decreases recorded as benefit to income tax provision — — (16) Increases recorded to income tax provision 8,173 2,784 — Valuation allowance as of end of year $ 10,957 $ 2,784 $ — The Company has not recorded any amounts for unrecognized tax benefits as of December 31, 2018 or 2017. The Company's policy is to record interest and penalties related to income taxes as part of its income tax provision. As of December 31, 2018 and 2017, the Company had no accrued interest or penalties related to uncertain tax positions and no amounts had been recognized in the Company's statement of operations and comprehensive loss. BPI files income tax returns in the U.S. and certain state jurisdictions. BPI's U.S. federal and state income tax returns are subject to tax examinations for the tax year ended December 31, 2015 and subsequent years. There are currently no income tax examinations pending.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Loss per Share	Net Loss per Share Net Loss per Share Basic and diluted net loss per share attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd. was calculated as follows: Year Ended December 31, 2018 2017 2016 Numerator: Net loss $ (240,922) $ (127,190) $ (63,534) Net (income) loss attributable to non-controlling interests — — 143 Accretion of beneficial conversion feature on Series A preferred shares — (12,006) — Net loss attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd. $ (240,922) $ (139,196) $ (63,677) Denominator: Weighted average common shares outstanding—basic and diluted 39,188,458 27,845,576 12,608,366 Net loss per share attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd.—basic and diluted $ (6.15) $ (5.00) $ (5.05) The Company's potential dilutive securities, which include stock options and warrants to purchase common shares, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common shareholders is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common shareholders for the periods indicated because including them would have had an anti-dilutive effect: Year Ended December 31, 2018 2017 2016 Options to purchase common shares 7,444,179 6,151,643 3,864,425 Warrants to purchase common shares 221,751 821,751 600,000 7,665,930 6,973,394 4,464,425 In addition to the potentially dilutive securities noted above, as of September 30, 2016, the Company agreed to issue warrants to purchase common shares to each of the Guarantor and Co-Guarantor of the Credit Agreement (see Note 9). In January 2017, the Company issued the warrants to the Guarantor and Co-Guarantor, pursuant to which each director received a warrant to purchase 107,500 common shares at an exercise price of $9.2911 per share. These warrants are included in the table above for the year ended December 31, 2017. In January 2018, the anti-dilution price protection provisions contained within the warrants issued to each of the guarantor and co-guarantor of the Credit Agreement expired, and upon expiration of the provision, the Company discontinued classification of these warrants as a liability. As such, these warrants are excluded above for the year ended December 31, 2018. The Company had also agreed under its agreements with AstraZeneca and BMS to issue common shares upon the achievement of specified milestones or upon the occurrence of specified events (see Notes 11 and 13). Because the necessary conditions for issuance of the shares had not been met as of December 31, 2016, the Company excluded these shares from the table above and from the calculation of diluted net loss per share for the year ended December 31, 2016. In May 2017, in connection with the completion of its IPO, the Company issued 538,149 common shares to AstraZeneca and 1,345,374 common shares to BMS in full satisfaction of its obligations to contingently issue equity securities pursuant to the license agreements. Accordingly, the table above does not include any shares related to the agreements with AstraZeneca and BMS for the year ended December 31, 2017.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Lease Agreement In December 2016, the Company entered into an assignment agreement to assume an operating lease for its office space in New Haven, Connecticut. The lease agreement expired in October 2018. In addition, the Company entered into a lease agreement for additional space which expired on June 30, 2018. The Company recorded rent expense for these leases as follows: Twelve Months Ended December 31, 2018 2017 2016 Rent expense $ 114 $ 73 $ — In August 2017, we entered into a lease agreement to consolidate our headquarters into a free standing building in New Haven, Connecticut, which we began occupying during the fourth quarter of 2018. The lease had a term of 85 months and commenced on January 1, 2018, with the ability to extend to 120 months. The Company had the option to purchase the property for $2,700 and executed that option in December 2018. The Company recorded the following for the lease agreement for its new headquarters during the construction period: Twelve Months Ended December 31, 2018 2017 2016 Rent expense $ 43 $ 75 $ — Capitalized costs 3,404 2,198 — License Agreements The Company has entered into license agreements with various parties under which it is obligated to make contingent and non-contingent payments (see Note 13). License agreements generally require the Company to pay annual maintenance fees and future payments upon the attainment of agreed upon development and/or commercial milestones. These agreements may also require minimum royalty payments based on sales of products developed from the applicable technologies, if any. The Company has submitted an investigational new drug application (“IND”) for BHV-3500, it's third generation CGRP receptor antagonist, and received approval to proceed and subsequently commenced a Phase 1 clinical trial in October of 2018 to permit later stage clinical trials. Per the BMS License Agreement, the Company is required to pay $2,000 to BMS on commencement of a Phase 1 clinical trial, and accordingly, the Company has recognized this liability in October of 2018. The payment obligation under the agreement is deferred until the earlier of the first approval or the discontinuation of the development of the Company's second generation CGRP receptor antagonist, rimegepant. Research Commitments The Company has entered into agreements with several CROs to provide services in connection with its preclinical studies and clinical trials. As of December 31, 2018, the Company had committed to minimum payments under these arrangements of $16,949, of which substantially all are due in the year ended December 31, 2019. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. The Company's amended and restated memorandum and articles of association also provide for indemnification of directors and officers in specific circumstances. To date, the Company has not incurred any material costs as a result of such indemnification provisions. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2018 or 2017. Legal Proceedings From time to time, in the ordinary course of business, the Company is subject to litigation and regulatory examinations as well as information gathering requests, inquiries and investigations. As of December 31, 2018, there were no matters which would have a material impact on the Company's financial results.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions License Agreement with Yale On September 30, 2013, the Company entered into the Yale Agreement with Yale (see Note 13). Yale is a related party because the Company's Chief Executive Officer is one of the inventors of the patents that the Company has licensed from Yale and, as such, is entitled to a specified share of the glutamate product-related royalty revenues that may be received by Yale under the Yale Agreement. As partial consideration for the license under the Yale Agreement, on September 30, 2013, the Company issued to Yale 250,000 common shares, representing 5.1% of the Company's then outstanding equity on a fully diluted basis. The fair value of the shares, totaling $152, was recognized as research and development expense at the time of issuance of the shares. During the years ended December 31, 2018, 2017 and 2016, the Company recognized no material research and development expense under the Yale Agreement, and as of December 31, 2018 and 2017, the Company owed no amounts to Yale. Relationship with Yale University Dr. Coric, the Company's Chief Executive Officer, previously served as an associate clinical professor of psychiatry at Yale. While previously employed by Yale, Dr. Coric was a co-inventor of some of the patents that the Company licenses from Yale. Under Yale's policies, as a co-inventor, Dr. Coric is entitled to receive a share of any royalties that the Company pays to Yale under the agreement with respect to the covered intellectual property and any proceeds from Yale's sale of the common shares the Company issued to Yale in connection with the license agreement. During 2017, Yale sold the common shares and, pursuant to Yale's policies, Dr. Coric received a payment from Yale of $0.6 million in March 2018. Guarantor and Co-Guarantor Warrants The Guarantor and Co-Guarantor of the Credit Agreement with Wells Fargo are each shareholders and members of the board of directors of the Company. The Company issued warrants to the Guarantor and Co-Guarantor in exchange for their respective guaranties (see Notes 9). The warrants were issued on January 26, 2017, pursuant to which each director received a warrant to purchase 107,500 common shares at an exercise price of $9.2911 per share. Kleo Pharmaceuticals, Inc. The Company has an investment in the common stock of Kleo (see Note 5). Kleo is a related party because the Company has determined that it exercises significant influence over the operating and financial policies of Kleo. In connection with its investment in Kleo, the Company received the right to designate two members of Kleo’s board of directors, who are the Chairman of the Company’s board of directors and another director of the Company. Also, the President and controlling stockholder of Kleo is a shareholder of the Company. The Company completed the last of four scheduled tranche purchases in January 2018, consisting of 1,375,000 shares for cash consideration of $1,375. In November 2018, the Company participated in Kleo's Series B funding raise. The Company purchased 1,420,818 shares for cash consideration of $5,000. As of December 31, 2018, the Company owned 41.9% of Kleo's outstanding capital stock. The Company has also entered into a clinical development master services agreement with Kleo to assist Kleo with clinical development. As of December 31, 2018, the Company had not performed any services or received any payments under this agreement. Biohaven Pharmaceuticals, Inc. BPI is a related party because the three founders, each of whom beneficially owned one-third of the equity of BPI prior to the Company's acquisition of BPI on December 31, 2016 (see Note 18), are shareholders of the Company and also serve as the Company's Chairman of the board of directors, Chief Executive Officer and Chief Medical Officer, respectively. On December 31, 2016, the Company acquired 100% of the capital stock of BPI for aggregate purchase consideration of $595 in the form of promissory notes to each of the former stockholders of BPI. In connection with the closing of the Company's IPO in May 2017, the notes were paid in full, including principal of $595, and accrued interest of $9. AstraZeneca The Company entered into an exclusive license agreement with AstraZeneca in October 2016. As part of the consideration under the agreement and in connection with the completion of the Series A First Closing in October 2016 and the completion of the IPO in May 2017, AstraZeneca received shares of the Company's stock (see Notes 1, 11 and 13). Bristol Myers-Squibb Company The Company entered into an exclusive license agreement with BMS in July 2016. As part of the consideration under the agreement and in connection with the completion of the Company's IPO in May 2017, BMS received shares of the Company's stock (see Notes 1, 11 and 13). The Company recorded $2,000 and $5,000 of research and development expense related to a payment made during the years ended December 31, 2018 and 2017 for the achievement of a specified milestone.

Acquisition of Biohaven Pharmac

Acquisition of Biohaven Pharmaceuticals, Inc.	12 Months Ended
	Dec. 31, 2018
Business Combinations [Abstract]
Acquisition of Biohaven Pharmaceuticals, Inc.	Acquisition of Biohaven Pharmaceuticals, Inc.The Company has historically outsourced all of the research and clinical development for its programs under a master services agreement (the "MSA") with BPI. The three founders of BPI, each of whom beneficially owned 33% of the equity of BPI prior to the Company's acquisition of BPI on December 31, 2016, are shareholders of the Company and also serve as the Company's Chairman of the board of directors, Chief Executive Officer, and Chief Medical Officer, respectively (see Note 17). BPI is a contract research organization whose only customer is the Company. Since its incorporation, substantially all of the operations of BPI have been performed in service to the Company under the terms of the MSA, and substantially all of the funding for the operations of BPI was provided by the Company. The Company determined that (i) it has the authority to direct the activities of BPI that most significantly impact the economics of the entity and (ii) the equity at risk in BPI is insufficient to finance its operations. As a result, the Company is deemed to have had a variable interest in BPI, and BPI is deemed to be a variable interest entity ("VIE") of which the Company is the primary beneficiary. Since the date of the Company's incorporation in September 2013, the Company has consolidated the results of BPI. Upon original consolidation, the Company applied purchase accounting by recording the fair values of BPI's assets acquired and liabilities assumed, which were determined to be zero because BPI had not yet commenced any operations. For the year ended December 31, 2016, 100% of the equity in BPI was reflected as a net loss attributable to non-controlling interest on the consolidated statement of operations and comprehensive loss. On December 31, 2016, the Company entered into stock purchase agreements with each of the stockholders of BPI, acquiring 100% of the issued and outstanding shares of BPI for aggregate purchase consideration of $595, and as a result, for periods subsequent to the acquisition, the Company no longer reports any non-controlling interest related to BPI. The Company funded the acquisition through the issuance of promissory notes to each of the former stockholders of BPI. In May 2017, in connection with the completion of the IPO, the notes became immediately due and payable, and the Company paid the notes, including principal and unpaid interest, in full. Because the Company consolidated BPI as a VIE prior to the acquisition, the acquisition of all of the capital stock of BPI did not result in a change of control for accounting purposes and was accounted for as an equity transaction. Accordingly, as of the acquisition date, the $86 carrying value of the non-controlling interest on December 31, 2016 was derecognized and the difference between the carrying value of the non-controlling interest of $86 and the purchase price of $595 was recorded as a $509 reduction to additional paid-in capital. There were no changes to this accounting treatment of BPI during the years ended December 31, 2018 and 2017. For the year ended December 31, 2016 when the Company consolidated BPI as a VIE, the Company recorded net income of $143 attributable to non-controlling interests.

Quarterly Financial Data (Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (Unaudited)	Quarterly Financial Data (Unaudited) The following information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information. Three Months Ended March 31, 2018 June 30, 2018 September 30, 2018 December 31, 2018 Operating expenses: Research and development $ 75,579 $ 29,052 $ 47,362 $ 37,958 General and administrative 7,857 9,064 7,574 10,108 Total operating expenses 83,436 38,116 54,936 48,066 Loss from operations (83,436) (38,116) (54,936) (48,066) Other income (expense): Interest expense (8) (13) (12) (5) Non-cash interest expense on liability related to sale of future royalties — (501) (5,633) (5,592) Change in fair value of warrant liability (1,182) — — — Loss from equity method investment (728) (641) (697) (742) Other (21) 27 (2) (151) Total other income (expense), net (1,939) (1,128) (6,344) (6,490) Loss before provision for income taxes (85,375) (39,244) (61,280) (54,556) Provision for income taxes 87 25 161 194 Net loss attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd. $ (85,462) $ (39,269) $ (61,441) $ (54,750) Net loss per share attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd.—basic and diluted $ (2.32) $ (1.01) $ (1.53) $ (1.34) Weighted average common shares outstanding—basic and diluted 36,793,090 38,942,545 40,147,735 40,938,709 Three Months Ended March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 Operating expenses: Research and development $ 10,740 $ 21,019 $ 34,996 $ 22,686 General and administrative 3,757 4,199 4,571 5,614 Total operating expenses 14,497 25,218 39,567 28,300 Loss from operations (14,497) (25,218) (39,567) (28,300) Other income (expense): Interest expense (305) (362) (239) — Interest income — — 10 (10) Change in fair value of warrant liability (454) (2,629) (2,426) 2,268 Change in fair value of derivative liability 289 223 — — Change in fair value of contingent equity liability (3,375) (9,707) — — Loss from equity method investment (218) (348) (638) (681) Total other income (expense), net (4,063) (12,823) (3,293) 1,577 Loss before provision for income taxes (18,560) (38,041) (42,860) (26,723) Provision for income taxes 193 399 55 359 Net loss and comprehensive loss $ (18,753) $ (38,440) $ (42,915) $ (27,082) Accretion of beneficial conversion feature on Series A preferred shares (4,000) (8,006) — — Net loss attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd. $ (22,753) $ (46,446) $ (42,915) $ (27,082) Net loss per share attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd.—basic and diluted $ (1.74) $ (1.78) $ (1.19) $ (0.75) Weighted average common shares outstanding—basic and diluted 13,088,861 26,038,192 35,930,698 35,984,111 During the quarter ended December 31, 2017, the Company recorded a $3,294 out-of-period adjustment to increase non-cash share-based compensation expense. Management has concluded that the error is not material to the current or prior period financial statements.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with 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 and the valuation of common shares, stock options, warrants, derivative instruments, contingent equity instruments, and non-cash interest expense on liability related to sale of future royalties. In addition, management’s assessment of the Company’s ability to continue as a going concern involves the estimation of the amount and timing of future cash inflows and outflows. 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 from those estimates.
Equity Method Investments, Including Related Impairment	Equity Method Investments, Including Related Impairment Investments in non-public companies in which the Company owns less than a 50% equity interest and where it has the ability to exercise significant influence over the operating and financial policies of the investee are accounted for using the equity method of accounting. The Company's proportionate share of the net income or loss of the equity method investment is included in other income (expense), net in the consolidated statement of operations and comprehensive loss and results in a corresponding adjustment to the carrying value of the investment on the consolidated balance sheet. Dividends received reduce the carrying value of the investment. An assessment of whether or not we have the power to direct activities that most significantly impact Kleo’s economic performance and to identify the party that obtains the majority of the benefits of the investment was performed as of December 31, 2018 and December 31, 2017, and will be performed as of each subsequent reporting date. After each of these assessments, we concluded that the activities that most significantly impact Kleo’s economic performance are the ability to direct its research activities, the ability to select vendors to perform the research, the ability to maintain research staff and the ability to raise additional funds, each of which are directed by Kleo. Based on the outcome of these assessments, we concluded that our investment in Kleo should be accounted for under the equity method. Changes related to this assessment could have a material impact on our financial statements. We also periodically review the carrying value of our investment in Kleo to determine if there has been an other-than-temporary decline in carrying value. A variety of factors are considered when determining if a decline in carrying value is other than temporary, including, among other factors, Kleo’s financial condition and business prospects, as well as our intent with regard to the investment. Changes related to the analysis of impairment of our investment in Kleo could have a material impact on our financial statements.
Property and Equipment	Property and Equipment Property and equipment are recorded at cost and depreciated or amortized using the straight-line method over the estimated useful lives of the respective assets. As of December 31, 2018, the Company's property and equipment consisted of an office building, office equipment and computer equipment. As of December 31, 2017, the Company's property and equipment consisted of office equipment and computer equipment, as well as construction in progress comprised of computer software and leasehold improvements. The fixed assets have the following useful lives: Building 30 years Office equipment 3 - 5 years Computer software 3 - 5 years Computer equipment 3 years 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 loss from operations. Expenditures for repairs and maintenance are
Fair Value Measurements	Fair Value Measurements Certain assets of the Company are carried at fair value under GAAP. 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. The Company's warrant liability, derivative liability and contingent equity liability are carried at fair value, based upon Level 3 inputs described above (see Note 3). The carrying values of other current assets, accounts payable and accrued expenses approximate their fair values due to the short-term nature of these assets and liabilities.
Segment Information	Segment Information The Company manages its operations as a single segment, the development of therapies targeting neurological diseases, for the purposes of assessing performance and making operating decisions. All of the Company's tangible assets are held in the 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, non-cash share-based compensation and benefits, third-party license fees, and external costs of vendors engaged to conduct clinical development activities and clinical trials as well as to manufacture clinical trial materials. Non-refundable prepayments for goods or services that will be used or rendered for future research and development activities are deferred and capitalized. Such amounts are recognized as an expense as the goods are delivered or the related services are performed, or until it is no longer expected that the goods will be delivered or the services rendered. The Company has entered into various research and development-related contracts. These agreements are 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. Certain judgments and 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.
Non-Cash Interest Expense on Liability Related to Sale of Future Royalties	Non-Cash Interest Expense on Liability Related to Sale of Future Royalties The Company accounted for the Funding Agreement with RPI as a liability financing, primarily because it has significant continuing involvement in generating the future revenue on which the royalties are based. The debt will be amortized under the effective interest rate method and, accordingly, the Company is recognizing non-cash interest expense over the estimated term of the Funding Agreement. The liability related to sale of future royalties, and the debt amortization, are based on the Company's current estimate of future royalties expected to be paid over the estimated term of the Funding Agreement. The Company will periodically assess the expected royalty payments and, if materially different than its previous estimate, will
Non-Cash Share-Based Compensation	Non-Cash Share-Based Compensation The Company measures stock options granted to employees, non-employees, and directors based on the fair value on the date of the grant and recognizes non-cash compensation expense of those awards, over the requisite service period, which is generally the vesting period of the respective award. Forfeitures are accounted for as they occur. Generally, the Company issues stock options with only service-based vesting conditions and records the expense for these awards using the straight-line method. The Company also issues, from time to time, stock options with performance-based vesting conditions and records the expense for these awards when the Company concludes that it is probable that the performance condition will be achieved. Effective July 1, 2018, the Company adopted Accounting Standards Update ("ASU") No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Non-employee Share-based Payment Accounting ("ASU 2018-07"), which sets out to simplify the accounting for non-employee share-based awards. The ASU expands the scope of Topic 718, Compensation-Stock Compensation, which currently only includes share-based payments issued to employees, to also include share-based payments issued to non-employees for goods and services. Consequently, the accounting for share-based payments to non-employees and employees is substantially aligned. ASU 2018-07 impacts the value at which share-based payments to non-employees is recognized. Prior to the adoption of ASU 2018-07 for share-based awards granted to non-employees, including consultants, non-cash compensation expense was recognized over the period during which services were rendered by such non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of the unvested awards were remeasured using the then-current fair value of the Company's common shares and updated assumption inputs in the Black-Scholes option-pricing model. After adoption of ASU 2018-07, the measurement date for non-employee awards is the date of the grant. The non-cash compensation expense for non-employees is recognized, without changes in the fair value of the award, over the requisite service period, which is the vesting period of the respective award. The non-cash compensation expense for non-employees was measured as of the adoption date of July 1, 2018, and this amount is the basis for prospective expense recognition. All of the Company's non-employee awards were previously measured as of June 30, 2018. Accordingly, no cumulative adjustment to beginning retained earnings was recorded as a result of the ASU 2018-07 adoption, as the measured value prior to adoption and the remeasured value on the date of adoption were materially the same. The Company classifies non-cash share-based compensation expense in its consolidated statement of operations and comprehensive loss in the same manner in which the award recipient's payroll costs are classified or in which the award recipient's service payments are classified. The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. The Company lacks a sufficient history of company-specific historical and implied volatility information for its shares. Therefore, it estimates its expected share price volatility based on the historical volatility of publicly traded peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded share price. The expected term of the Company's stock options has been determined utilizing the "simplified" method for awards that qualify as "plain-vanilla" options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends on common shares and does not expect to pay any cash dividends in the foreseeable future.
Warrant Liability	Warrant Liability In connection with entering into a credit agreement, the Company issued warrants to purchase common shares to two of the Company's directors in connection with a guarantee of its obligations under the agreement (see Note 9). The Company classifies the warrants as a liability on its consolidated balance sheet because each warrant represents a freestanding financial instrument that is not indexed to the Company's own shares. The warrant liability was initially recorded at fair value upon entering into the credit agreement and is subsequently remeasured to fair value at each reporting date. Changes in the fair value of the warrant liability are recognized as a component of other income (expense), net in the consolidated statement of operations and comprehensive loss. Changes in the fair value of the warrant liability will continue to be recognized until the
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 the Company's 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 its deferred tax assets will be recovered from future taxable income and, to the extent it 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 provision for income taxes includes the effects of applicable tax reserves, or unrecognized tax benefits, as well as the related net interest and penalties.
Net Income (Loss) per Share	Net Income (Loss) per Share The Company follows the two-class method when computing net income (loss) per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common shareholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. Net income (loss) per share attributable to common shareholders is calculated based on net income (loss) attributable to Biohaven Pharmaceutical Holding Company Ltd. and excludes net income (loss) attributable to non-controlling interests for relevant periods. Basic net income (loss) per share attributable to common shareholders is computed by dividing the net income (loss) attributable to common shareholders by the weighted average number of common shares outstanding for the period. Diluted net income (loss) attributable to common shareholders is computed by adjusting net income (loss) attributable to common shareholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common shareholders is computed by dividing the diluted net income (loss) attributable to common shareholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares. For purpose of this calculation, outstanding options, warrants to purchase common shares, convertible preferred shares and contingently issuable equity are considered potential dilutive common shares. The Company's convertible preferred shares contractually entitled the holders of such shares to participate in dividends but contractually did not require the holders of such shares to participate in losses of the Company. In periods in which the Company reports a net loss attributable to common shareholders, diluted net loss per share attributable to common shareholders is the same as basic net loss per share attributable to common shareholders, since potentially dilutive common shares are considered to be anti-dilutive.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements Effective January 1, 2018, the Company adopted ASU No. 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017-09”), which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. The adoption of this new guidance had no impact on the Company's financial position or operating results. Effective January 1, 2018, the Company adopted ASU 2016-18, “Statement of Cash Flows (Topic 230): Restricted Cash,” which requires entities to show the change in the total of cash, cash equivalents, restricted cash and restricted cash equivalents within the statement of cash flows. The guidance is effective retrospectively. As a result, the Company retrospectively included restricted cash in the beginning cash for the twelve months ended December 31, 2017 on the consolidated statement of cash flows, and no longer separately presents transfers between unrestricted cash and restricted cash. The Company did not have restricted cash as of December 31, 2018 or December 31, 2017. Effective January 1, 2018, the Company adopted ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“ASU 2016-15”), which addresses diversity in practice in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. The adoption of this new guidance had no impact on the Company's financial position or operating results. Effective July 1, 2018, the Company adopted Accounting Standards Update ("ASU") No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Non-employee Share-based Payment Accounting ("ASU 2018-07"), which sets out to simplify the accounting for non-employee share-based awards. The ASU expands the scope of Topic 718, Compensation-Stock Compensation, which currently only includes share-based payments issued to employees, to also include share-based payments issued to non-employees for goods and services. Consequently, the accounting for share-based payments to non-employees and employees is substantially aligned. ASU 2018-07 impacts the value at which share-based payments to non-employees is recognized. Prior to the adoption of ASU 2018-07 for share-based awards granted to non-employees, including consultants, non-cash compensation expense was recognized over the period during which services were rendered by such non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of the unvested awards were remeasured using the then-current fair value of the Company's common shares and updated assumption inputs in the Black-Scholes option-pricing model. After adoption of ASU 2018-07, the measurement date for non-employee awards is the date of the grant. The non-cash compensation expense for non-employees is recognized, without changes in the fair value of the award, over the requisite service period, which is the vesting period of the respective award. The non-cash compensation expense for non-employees was measured as of the adoption date of July 1, 2018, and this amount is the basis for prospective expense recognition. All of the Company's non-employee awards were previously measured as of June 30, 2018. Accordingly, no cumulative adjustment to beginning retained earnings was recorded as a result of the ASU 2018-07 adoption, as the measured value prior to adoption and the remeasured value on the date of adoption were materially the same.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease, respectively. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. ASU 2016-02 (Accounting Standards Codification (“ASC”) Topic 842) supersedes the previous leases standard, ASC 840, Leases. The standard is effective for public entities for annual periods beginning after December 15, 2018 and for interim periods within those fiscal years. Subsequently, in July of 2018, the FASB issued ASU No. 2018-10, Codification Improvements to Topic 842, Leases (“ASU 2018-10”), and ASU No. 2018-11, Leases (Topic 842): Targeted Improvements (“ASU 2018-11”), both of which clarify and enhance the certain amendments made in ASU 2016-02 and will be adopted in conjunction with ASU 2016-02. The Company intends to elect the package of practical expedients and is currently evaluating the impact that the adoption of ASU 2016-02 will have on its consolidated financial statements.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Schedule of useful lives of fixed assets	The fixed assets have the following useful lives: Building 30 years Office equipment 3 - 5 years Computer software 3 - 5 years Computer equipment 3 years

Fair Value of Financial Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Summary of the Company's financial assets and liabilities measured at fair value on a recurring basis and indicate the level of the fair value hierarchy	The following table presents information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values: Fair Value Measurement as of December 31, 2017 Using: Level 1 Level 2 Level 3 Total Liabilities: Warrant liability $ — $ — $ 4,021 $ 4,021 $ — $ — $ 4,021 $ 4,021
Summary of roll forward of aggregate fair values of the Company's warrant liability, derivative liability and contingent equity liability	The following table provides a roll forward of the aggregate fair values of the Company's warrant liability, derivative liability and contingent equity liability, for which fair value is determined by Level 3 inputs: Warrant Liability Derivative Liability Contingent Equity Liability December 31, 2016 780 512 18,938 Change in fair value 3,241 (512) 13,082 Issuance of common shares in settlement of contingent equity liability — — (32,020) December 31, 2017 4,021 — — Change in fair value 1,182 — — Reclassification to equity (5,203) — — Balance as at December 31, 2018 $ — $ — $ —

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2018
Prepaid Expense and Other Assets, Current [Abstract]
Schedule of prepaid expenses and other current assets	Prepaid expenses and other current assets consisted of the following: December 31, 2018 2017 Prepaid clinical trial costs $ 7,210 $ 4,642 Prepaid insurance 393 455 Other 487 100 $ 8,090 $ 5,197

Equity Method Investment (Table

Equity Method Investment (Tables)	12 Months Ended
	Dec. 31, 2018
Equity Method Investments and Joint Ventures [Abstract]
Schedule of carrying value of equity method investment	The following table provides a roll forward of the carrying value of the Company's equity method investment: Carrying Value Balance as at December 31, 2016 $ 2,753 Purchase of Kleo common stock 6,979 Loss recognized in connection with equity method investment (1,885) Balance as at December 31, 2017 7,847 Purchases of Kleo common stock 6,375 Loss recognized in connection with equity method investment (2,808) Balance as at December 31, 2018 $ 11,414
Schedule of equity investments and summarized financial information for equity investees	Summarized financial information for Kleo is as follows: December 31, 2018 2017 Current assets $ 23,762 $ 8,388 Total assets $ 24,048 $ 8,746 Current liabilities $ 1,598 $ 1,415 Total liabilities $ 2,054 $ 4,201 Year Ended December 31, 2018 2017 2016 Revenue $ — $ — $ — Loss from operations $ (6,501) $ (5,646) $ (3,764) Net loss $ (6,334) $ (5,658) $ (3,727)

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Schedule of property and equipment, net	Property and equipment, net consisted of the following: December 31, 2018 2017 Building and Land $ 2,200 $ — Building Improvements 3,210 — Computer Hardware 420 163 Furniture & Fixtures 280 — Office Equipment 441 26 $ 6,551 $ 189 Accumulated depreciation (303) (43) Construction in progress — 2,198 $ 6,248 $ 2,344

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2018
Payables and Accruals [Abstract]
Schedule of accrued expenses	Accrued expenses consisted of the following: December 31, 2018 2017 Accrued employee compensation and benefits $ 108 $ 89 Accrued clinical trial costs 6,753 3,582 Accrued professional fees 1,636 390 Lease liability — 362 Other 285 285 $ 8,782 $ 4,708

Liabilities Related to Sale o_2

Liabilities Related to Sale of Future Royalties (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule of activity within liability related to sale of future royalties	The following table shows the activity within the liability related to sale of future royalties for the twelve months ended December 31, 2018: Liability Related to Sale of Future Royalties Transaction date balance $ 106,047 Non-cash interest expense recognized, net of transaction cost amortization 11,726 Balance at December 31, 2018 117,773 Less: Unamortized transaction costs (258) Carrying value at December 31, 2018 $ 117,515

Warrants (Tables)

Warrants (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule of warrant liability and related income (expense)	The following table provides income (expense) related to the warrant liability which the Company records net within other income (expense) in the consolidated statements of operations: Twelve Months Ended December 31, 2018 2017 2016 Income (expense) from change in fair value of warrant liability $ (1,182) $ (3,241) $ 154

Share-Based Compensation (Table

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of assumptions used to determine the grant-date fair value of stock options granted	The assumptions that the Company used to determine the grant-date fair value of stock options granted to employees and directors under the 2014 Plan and the 2017 Plan (collectively, the "Plans") were as follows, presented on a weighted average basis: Year Ended December 31, 2018 2017 2016 Risk-free interest rate 2.91 % 2.10 % 2.19 % Expected term (in years) 6.25 6.02 5.75 Expected volatility 73.03 % 73.26 % 70.58 % Expected dividend yield — % — % — % Exercise price $ 32.35 $ 18.47 $ 9.29 The assumptions that the Company used to determine the grant-date fair value of stock options granted to non-employees under the Plans were as follows, presented on a weighted average basis: Year Ended December 31, 2018 2017 2016 Risk-free interest rate 3.06 % 2.35 % 2.54 % Expected term (in years) 10 10 10 Expected volatility 74.50 % 71.12 % 67.16 % Expected dividend yield — % — % — % Exercise price $ 32.42 $ 18.23 $ 9.29
Schedule of stock option activity	Stock option activity under the Plans is summarized as follows: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2017 6,151,643 $ 9.97 8.42 $ 107,072 Granted 1,955,000 $ 32.35 9.70 Exercised (577,288) $ 6.26 Forfeited (85,176) $ 22.54 Outstanding as of December 31, 2018 7,444,179 $ 15.99 8.05 $ 156,518 Options exercisable as of December 31, 2018 3,854,598 $ 6.26 6.92 $ 118,452 Options unvested as of December 31, 2018 3,589,581 $ 26.46 9.26 $ 38,066
Schedule of non-cash share-based compensation expense	Non-cash share-based compensation expense was classified in the consolidated statements of operations and comprehensive loss as follows: Year Ended December 31, 2018 2017 2016 Research and development expenses $ 8,371 $ 6,933 $ 2,382 General and administrative expenses 8,554 6,306 2,221 $ 16,925 $ 13,239 $ 4,603

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of income (loss) before provision for income taxes	Income (loss) before provision for income taxes consisted of the following: Year Ended December 31, 2018 2017 2016 BVI $ (246,829) $ (130,359) $ (63,677) Foreign (U.S.) 6,374 4,175 233 Loss before provision for income taxes $ (240,455) $ (126,184) $ (63,444)
Schedule of provision for income taxes	The provision for income taxes consisted of the following: Year Ended December 31, 2018 2017 2016 Current income tax provision: BVI $ — $ — $ — Foreign (U.S. federal and state) 467 997 99 Total current income tax provision 467 997 99 Deferred income tax provision (benefit): BVI — — — Foreign (U.S. federal and state) — 9 (9) Total deferred income tax provision (benefit) — 9 (9) Total provision for income taxes $ 467 $ 1,006 $ 90
Schedule of reconciliation of the BVI statutory income tax rate to the company's effective income tax rate	A reconciliation of the BVI statutory income tax rate of 0% to the Company's effective income tax rate is as follows: Year Ended December 31, 2018 2017 2016 BVI statutory income tax rate .0 % .0 % .0 % Foreign tax rate differential .6 1.2 .1 Tax Credits (3.5) (2.7) .0 Change in valuation allowance 3.4 2.2 .0 Other (.3) .1 .0 Effective income tax rate .2 % .8 % .1 %
Schedule of net deferred taxes assets (liabilities)	Net deferred tax assets (liabilities) consisted of the following: December 31, 2018 2017 Deferred tax assets: Foreign net operating loss carryforwards $ — $ — Tax credits 11,396 2,790 Other 1 1 Total deferred tax assets 11,397 2,791 Deferred tax liabilities: Other (440) (7) Total deferred tax liabilities (440) (7) Valuation allowance (10,957) (2,784) Net deferred tax assets $ — $ —
Summary of changes in valuation allowance for deferred tax assets	Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2018 and 2017 were due primarily to generation of excess credits and were as follows: Year Ended December 31, 2018 2017 2016 Valuation allowance as of beginning of year $ 2,784 $ — $ 16 Decreases recorded as benefit to income tax provision — — (16) Increases recorded to income tax provision 8,173 2,784 — Valuation allowance as of end of year $ 10,957 $ 2,784 $ —

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Schedule of basic and diluted net loss per share	Basic and diluted net loss per share attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd. was calculated as follows: Year Ended December 31, 2018 2017 2016 Numerator: Net loss $ (240,922) $ (127,190) $ (63,534) Net (income) loss attributable to non-controlling interests — — 143 Accretion of beneficial conversion feature on Series A preferred shares — (12,006) — Net loss attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd. $ (240,922) $ (139,196) $ (63,677) Denominator: Weighted average common shares outstanding—basic and diluted 39,188,458 27,845,576 12,608,366 Net loss per share attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd.—basic and diluted $ (6.15) $ (5.00) $ (5.05)
Schedule of potentially anti-dilutive securities excluded from calculation of diluted net loss per share	The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common shareholders for the periods indicated because including them would have had an anti-dilutive effect: Year Ended December 31, 2018 2017 2016 Options to purchase common shares 7,444,179 6,151,643 3,864,425 Warrants to purchase common shares 221,751 821,751 600,000 7,665,930 6,973,394 4,464,425

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Schedule of rent expense	The Company recorded rent expense for these leases as follows: Twelve Months Ended December 31, 2018 2017 2016 Rent expense $ 114 $ 73 $ —
Schedule of lease arrangement for new headquarters	The Company recorded the following for the lease agreement for its new headquarters during the construction period: Twelve Months Ended December 31, 2018 2017 2016 Rent expense $ 43 $ 75 $ — Capitalized costs 3,404 2,198 —

Quarterly Financial Data (Una_2

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Summary of quarterly financial data	The following information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information. Three Months Ended March 31, 2018 June 30, 2018 September 30, 2018 December 31, 2018 Operating expenses: Research and development $ 75,579 $ 29,052 $ 47,362 $ 37,958 General and administrative 7,857 9,064 7,574 10,108 Total operating expenses 83,436 38,116 54,936 48,066 Loss from operations (83,436) (38,116) (54,936) (48,066) Other income (expense): Interest expense (8) (13) (12) (5) Non-cash interest expense on liability related to sale of future royalties — (501) (5,633) (5,592) Change in fair value of warrant liability (1,182) — — — Loss from equity method investment (728) (641) (697) (742) Other (21) 27 (2) (151) Total other income (expense), net (1,939) (1,128) (6,344) (6,490) Loss before provision for income taxes (85,375) (39,244) (61,280) (54,556) Provision for income taxes 87 25 161 194 Net loss attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd. $ (85,462) $ (39,269) $ (61,441) $ (54,750) Net loss per share attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd.—basic and diluted $ (2.32) $ (1.01) $ (1.53) $ (1.34) Weighted average common shares outstanding—basic and diluted 36,793,090 38,942,545 40,147,735 40,938,709 Three Months Ended March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 Operating expenses: Research and development $ 10,740 $ 21,019 $ 34,996 $ 22,686 General and administrative 3,757 4,199 4,571 5,614 Total operating expenses 14,497 25,218 39,567 28,300 Loss from operations (14,497) (25,218) (39,567) (28,300) Other income (expense): Interest expense (305) (362) (239) — Interest income — — 10 (10) Change in fair value of warrant liability (454) (2,629) (2,426) 2,268 Change in fair value of derivative liability 289 223 — — Change in fair value of contingent equity liability (3,375) (9,707) — — Loss from equity method investment (218) (348) (638) (681) Total other income (expense), net (4,063) (12,823) (3,293) 1,577 Loss before provision for income taxes (18,560) (38,041) (42,860) (26,723) Provision for income taxes 193 399 55 359 Net loss and comprehensive loss $ (18,753) $ (38,440) $ (42,915) $ (27,082) Accretion of beneficial conversion feature on Series A preferred shares (4,000) (8,006) — — Net loss attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd. $ (22,753) $ (46,446) $ (42,915) $ (27,082) Net loss per share attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd.—basic and diluted $ (1.74) $ (1.78) $ (1.19) $ (0.75) Weighted average common shares outstanding—basic and diluted 13,088,861 26,038,192 35,930,698 35,984,111

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation (Details)	12 Months Ended
	Dec. 31, 2016founder
Related Party Transactions
Number of founders	3
Equity share percentage owned by each founders	33.33%
Ownership interest acquired	100.00%
Net loss attributable to non-controlling interest	100.00%

Nature of the Business and Ba_3

Nature of the Business and Basis of Presentation - Stock Split & Public Offering (Details) $ / shares in Units, $ in Thousands	May 11, 2017USD ($)shares	May 09, 2017USD ($)$ / sharesshares	Jun. 30, 2018USD ($)shares	May 31, 2017USD ($)$ / sharesshares	Oct. 31, 2016	Jul. 31, 2016USD ($)$ / sharesshares	Feb. 29, 2016USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)shares	Apr. 21, 2017shares
Stock Split
Stock Split					500
Public Offering
Common stock sold (shares) | shares	1,883,523		1,111,111			1,090,500	429,000
Fair value of common share | $ / shares						$ 7.70	$ 7
Other offering expenses			$ 111			$ 97	$ 23	$ 2,987	$ 5,068	$ 1,507
Conversion of Series A convertible preferred shares to common shares (shares) | shares				9,358,560
Aggregate net proceeds from the IPO and underwriters issuance				$ 176,128
Common shares value issued								$ 230,339	$ 176,128	$ 11,279
Common stock, authorized (shares) | shares				200,000,000				200,000,000	200,000,000	38,000,000	50,000,000
Common stock, par value (in dollars per share) | $ / shares				$ 0				$ 0	$ 0
Preferred stock, shares authorized | shares				10,000,000
Preferred stock, par value (in dollars per share) | $ / shares				$ 0
IPO
Public Offering
Common stock sold (shares) | shares		9,900,000		9,900,000
Fair value of common share | $ / shares		$ 17		$ 17
Proceeds from issuance of common shares upon completion of initial public offering, net of underwriting commissions and discounts		$ 152,651		$ 152,651
Underwriting discounts and commissions		11,781		11,781
Other offering expenses		$ 3,868		$ 3,868
Over allotment option
Public Offering
Common stock sold (shares) | shares	1,485,000			1,485,000
Proceeds from issuance of common shares upon completion of initial public offering, net of underwriting commissions and discounts	$ 23,478			$ 23,478
Underwriting discounts and commissions	1,767			$ 1,767
Aggregate net proceeds from the IPO and underwriters issuance	$ 176,128
Over allotment option | BMS and AstraZeneca
Public Offering
Common stock sold (shares) | shares	1,883,523
Common shares value issued	$ 0

Nature of the Business and Ba_4

Nature of the Business and Basis of Presentation - Going Concern (Details) - USD ($) $ / shares in Units, $ in Thousands	May 11, 2017	Dec. 31, 2018	Jun. 30, 2018	Jul. 31, 2016	Feb. 29, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Net losses						$ 27,082	$ 42,915	$ 38,440	$ 18,753	$ 240,922	$ 127,190	$ 63,534
Accumulated deficit		$ 443,568				202,646				443,568	202,646
Cash		$ 264,249				$ 131,468				264,249	131,468	23,692	$ 1,460
Notes Payable
Common stock sold (shares)	1,883,523		1,111,111	1,090,500	429,000
Proceeds from sale of common stock			$ 49,889	$ 8,299	$ 2,980					$ 190,125	$ 179,996	$ 11,399
Underwritten Public Offering
Notes Payable
Common stock sold (shares)		3,859,060
Common stock sold, price per share (in dollars per share)		$ 37.25								$ 37.25
Proceeds from sale of common stock		$ 134,485
RPI Funding Agreement
Notes Payable
Proceeds from royalty agreement			$ 100,000
RPI Purchase Agreement
Notes Payable
Common stock sold (shares)			1,111,111
Common stock sold, price per share (in dollars per share)			$ 45
Proceeds from sale of common stock			$ 50,000
RPI Agreement
Notes Payable
Proceeds from royalty agreement			149,623
Issuance costs			$ 377

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Details) - director	Aug. 30, 2016	Dec. 31, 2018
Warrant Liability
Number of directors to whom the Company agreed to issue warrants	2	2
Building
Property and Equipment
Estimated useful life		30 years
Leasehold improvements
Property and Equipment
Estimated useful life		10 years
Computer equipment
Property and Equipment
Estimated useful life		3 years
Minimum | Office Equipment
Property and Equipment
Estimated useful life		3 years
Minimum | Computer software
Property and Equipment
Estimated useful life		3 years
Maximum | Office Equipment
Property and Equipment
Estimated useful life		5 years
Maximum | Computer software
Property and Equipment
Estimated useful life		5 years

Fair Value of Financial Asset_3

Fair Value of Financial Assets and Liabilities - Measured on a Recurring Basis (Details) - Recurring - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of financial liabilities	$ 0	$ 4,021,000
Fair value of financial assets	$ 0
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of financial liabilities		0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of financial liabilities		0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of financial liabilities		4,021,000
Warrant Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrant liability		4,021,000
Warrant Liability | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrant liability		0
Warrant Liability | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrant liability		0
Warrant Liability | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrant liability		$ 4,021,000

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities - Narrative (Details)	Aug. 30, 2016director	May 31, 2017shares	Feb. 28, 2017USD ($)shares	Oct. 31, 2016USD ($)tranche$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesdirector	Dec. 31, 2017USD ($)shares	Dec. 31, 2016USD ($)shares	Oct. 31, 2017USD ($)	Jul. 31, 2016$ / shares	Feb. 29, 2016$ / shares
Fair value of Financial Assets and Liabilities
Number of directors to whom the Company agreed to issue warrants | director	2				2
Shares issued in connection with contingent equity liability (shares) | shares		9,358,560
Issuance price per share (in dollars per share) | $ / shares									$ 7.70	$ 7
Recognized fair value of BCF as a reduction to series A preferred shares and corresponding adjustment to additional paid in capital					$ 0	$ (12,006,000)	$ 0
BMS
Fair value of Financial Assets and Liabilities
Value of the contingently issuable equity							$ 18,750,000
Shares issued in connection with contingent equity liability (shares) | shares		1,345,374
BMS | Maximum
Fair value of Financial Assets and Liabilities
Expected timing of event that was most probable of triggering the issuance of shares							1 year
AstraZeneca
Fair value of Financial Assets and Liabilities
Shares issued in connection with contingent equity liability (shares) | shares		538,149
Number of tranches in which contingently issuable equity is issuable | tranche				2
Fixed dollar amount of tranches				$ 5,000,000
Total amount of tranches				$ 10,000,000
AstraZeneca | Second tranche
Fair value of Financial Assets and Liabilities
Value of the contingently issuable equity							$ 7,500,000
Fair value of the contingent equity liability							$ 4,875,000
AstraZeneca | Second tranche | Maximum
Fair value of Financial Assets and Liabilities
Expected timing of event that was most probable of triggering the issuance of shares							1 year
Series A Convertible Preferred Shares
Fair value of Financial Assets and Liabilities
Shares issued in connection with contingent equity liability (shares) | shares		9,358,560
Preferred stock issued and sold (shares) | shares			4,305,182	4,305,209		4,305,182	4,305,209
Aggregate fair value						$ 38,666,000	$ 38,270,000
Recognized fair value of BCF as a reduction to series A preferred shares and corresponding adjustment to additional paid in capital			$ 12,006,000			$ 12,006,000
Shares issued on conversion of preferred stock (shares) | shares		9,358,560
Series A Convertible Preferred Shares | AstraZeneca
Fair value of Financial Assets and Liabilities
Preferred stock issued and sold (shares) | shares				538,150
Series A Convertible Preferred Shares | AstraZeneca | First tranche
Fair value of Financial Assets and Liabilities
Preferred stock issued and sold (shares) | shares				538,150
Aggregate fair value				$ 5,000,000
Issuance price per share (in dollars per share) | $ / shares				$ 9.2911
Expected Dividend Yield
Fair value of Financial Assets and Liabilities
Measurement input for warrant liability | $ / shares					0
Probability Of Triggering Event | BMS
Fair value of Financial Assets and Liabilities
Measurement input for contingent equity liability							0.75
Probability Of Triggering Event | AstraZeneca | Second tranche
Fair value of Financial Assets and Liabilities
Measurement input for contingent equity liability							0.65
Discount Rate | BMS
Fair value of Financial Assets and Liabilities
Measurement input for contingent equity liability							0
Discount Rate | AstraZeneca | Second tranche
Fair value of Financial Assets and Liabilities
Measurement input for contingent equity liability							0
Derivative Liability
Fair value of Financial Assets and Liabilities
Derivative liability					$ 0			$ 0

Fair Value of Financial Asset_5

Fair Value of Financial Assets and Liabilities - Rollforward of Liabilities Measured by Level 3 Inputs (Details) - USD ($) $ in Thousands	Jan. 26, 2018	Dec. 31, 2018	Dec. 31, 2017
Roll Forward of Liabilities Measured by Level 3 Inputs:
Issuance of common shares in settlement of contingent equity liability			$ (32,020)
Reclassification to equity	$ (5,203)	$ (5,203)
Warrant Liability
Roll Forward of Liabilities Measured by Level 3 Inputs:
Beginning balance		4,021	780
Change in fair value		1,182	3,241
Issuance of common shares in settlement of contingent equity liability			0
Reclassification to equity		5,203
Ending balance		0	4,021
Derivative Liability
Roll Forward of Liabilities Measured by Level 3 Inputs:
Beginning balance		0	512
Change in fair value		0	(512)
Issuance of common shares in settlement of contingent equity liability			0
Reclassification to equity		0
Ending balance		0	0
Contingent equity liability
Roll Forward of Liabilities Measured by Level 3 Inputs:
Beginning balance		0	18,938
Change in fair value		0	13,082
Issuance of common shares in settlement of contingent equity liability			(32,020)
Reclassification to equity		0
Ending balance		$ 0	$ 0

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Prepaid Expense and Other Assets, Current [Abstract]
Prepaid clinical trial costs	$ 7,210	$ 4,642
Prepaid insurance	393	455
Other	487	100
Total prepaid expenses and other current assets	$ 8,090	$ 5,197

Equity Method Investment - Stoc

Equity Method Investment - Stock purchase agreement with Kleo (Details) $ / shares in Units, $ in Thousands	Aug. 31, 2016shares	Aug. 29, 2016tranchedirector$ / sharesshares	Nov. 30, 2018USD ($)shares	Jan. 31, 2018USD ($)	Oct. 31, 2017USD ($)shares	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)shares	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)shares	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Equity Method Investment [Line Items]
Cash payment for purchase of equity method investment																$ 6,375	$ 6,627	$ 3,000
Loss from equity method investment								$ (742)	$ (697)	$ (641)	$ (728)	$ (681)	$ (638)	$ (348)	$ (218)	(2,808)	(1,885)	(247)
Carrying value of investment in Kleo								$ 11,414				$ 7,847				11,414	7,847
Kleo
Equity Method Investment [Line Items]
Shares of Kleo common stock purchased | shares			1,420,818		2,049,543
Cash payment for purchase of equity method investment			$ 5,000		$ 2,253											$ 6,375	$ 6,979
Shares of Kleo common stock purchased from Chief Executive Officer | shares							500,000								500,000
Cash payment for purchase of Kleo officer and stockholder shares							$ 250								$ 250
Issuance of common shares for purchase of Kleo office and stockholder shares (in shares) | shares							32,500								32,500
Ownership percentage				46.60%				41.90%				43.30%				41.90%	43.30%
Loss from equity method investment																$ (2,808)	$ (1,885)
Carrying value of investment in Kleo								$ 11,414				$ 7,847				$ 11,414	$ 7,847	$ 2,753
Kleo Stock Purchase Agreement | Kleo
Equity Method Investment [Line Items]
Shares of Kleo common stock purchased | shares	3,000,000
Additional shares to be purchased per stock purchase agreement (shares) | shares		5,500,000
Price per share | $ / shares		$ 1
Number of equal tranches of shares to be purchased | tranche		4
Shares purchased in equal tranches | shares		1,375,000
Period of time shares begin to be purchased in equal tranches after the initial closing		6 years
Period of time shares are purchased after the initial purchase		3 years
Number to designate to Kleo's board of directors | director		2
Cash payment for purchase of equity method investment				$ 1,375	$ 1,375	$ 1,375	$ 1,375

Equity Method Investment - Carr

Equity Method Investment - Carrying value and Summarized financial information for Kleo (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended								12 Months Ended
	Nov. 30, 2018	Oct. 31, 2017	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
Carrying value of equity method investment
Beginning balance						$ 7,847					$ 7,847
Purchase of Kleo common stock											6,375	$ 6,627	$ 3,000
Loss recognized in connection with equity method investment			$ (742)	$ (697)	$ (641)	(728)	$ (681)	$ (638)	$ (348)	$ (218)	(2,808)	(1,885)	(247)
Ending balance			11,414				7,847				11,414	7,847
Kleo
Carrying value of equity method investment
Beginning balance						$ 7,847				$ 2,753	7,847	2,753
Purchase of Kleo common stock	$ 5,000	$ 2,253									6,375	6,979
Loss recognized in connection with equity method investment											(2,808)	(1,885)
Ending balance			11,414				7,847				11,414	7,847	2,753
Balance Sheet
Current assets			23,762				8,388				23,762	8,388
Total assets			24,048				8,746				24,048	8,746
Current liabilities			1,598				1,415				1,598	1,415
Total liabilities			$ 2,054				$ 4,201				2,054	4,201
Statement of Operations
Revenue											0	0	0
Loss from Operations											(6,501)	(5,646)	(3,764)
Net loss											$ (6,334)	$ (5,658)	$ (3,727)

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property and Equipment, Net
Property and equipment, Gross	$ 6,551	$ 189
Accumulated depreciation	(303)	(43)
Construction in Progress, Gross	0	2,198
Property and Equipment, Net	6,248	2,344
Buildings and Land
Property and Equipment, Net
Property and equipment, Gross	2,200	0
Building Improvements
Property and Equipment, Net
Property and equipment, Gross	3,210	0
Computer Hardware
Property and Equipment, Net
Property and equipment, Gross	420	163
Furniture and Fixtures
Property and Equipment, Net
Property and equipment, Gross	280	0
Office Equipment
Property and Equipment, Net
Property and equipment, Gross	$ 441	$ 26

Property and Equipment, Net - N

Property and Equipment, Net - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property and Equipment, Net
Depreciation expense		$ 261	$ 35	$ 5
Finance leased asset included in construction in progress	$ 0	$ 0	$ 1,787
New Headquarters, New Haven, CT
Property and Equipment, Net
Purchase of property upon execution of option	$ 2,700

Accrued Expenses (Details)

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued expenses
Accrued employee compensation and benefits	$ 108	$ 89
Accrued clinical trial costs	6,753	3,582
Accrued professional fees	1,636	390
Lease liability	0	362
Other	285	285
Total accrued expenses	$ 8,782	$ 4,708

Liabilities Related to Sale o_3

Liabilities Related to Sale of Future Royalties (Details) - USD ($) $ / shares in Units, $ in Thousands	May 11, 2017	Jun. 30, 2018	Jul. 31, 2016	Feb. 29, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Notes Payable
Common stock sold (shares)	1,883,523	1,111,111	1,090,500	429,000
Proceeds from sale of common stock		$ 49,889	$ 8,299	$ 2,980	$ 190,125	$ 179,996	$ 11,399
Transaction consideration allocated to liability		106,047
RPI Funding Agreement
Notes Payable
Proceeds from royalty agreement		$ 100,000
Percentage of revenue participation rights available for repurchase (percent)		100.00%
Price of revenue participation rights available for repurchase		$ 155,000
Effective interest rate (percent)					22.00%
RPI Purchase Agreement
Notes Payable
Common stock sold (shares)		1,111,111
Common stock sold, price per share (in dollars per share)		$ 45
Proceeds from sale of common stock		$ 50,000
RPI Agreement
Notes Payable
Proceeds from royalty agreement		149,623
Transaction consideration allocated to equity		43,953
Transaction costs		$ 377
Revenue Participation Right, tranche one | RPI Funding Agreement
Notes Payable
Participation rate for revenue participation right (percent)		2.10%
Revenue Participation Right, tranche one | RPI Funding Agreement | Maximum
Notes Payable
Annual global net sales threshold for revenue participation right		$ 1,500,000
Revenue Participation Right, tranche two | RPI Funding Agreement
Notes Payable
Participation rate for revenue participation right (percent)		1.50%

Liability Related to Sale of Fu

Liability Related to Sale of Future Royalties - Carrying Value and Related Activity (Details) - USD ($) $ in Thousands	6 Months Ended
	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
Liability related to sale of future royalties, transaction date balance		$ 106,047
Non-cash interest expense recognized, net of transaction cost amortization	$ 11,726
Liability related to sale of future royalties, gross	117,773
Less: Unamortized transaction costs	(258)
Liability related to sale of future royalties, net	$ 117,515		$ 0

Warrants - Credit Agreement (De

Warrants - Credit Agreement (Details)	Aug. 31, 2017USD ($)	Aug. 30, 2016USD ($)director	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 ($)director	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Short-term Debt [Line Items]
Proceeds from borrowings											$ 0	$ 0	$ 5,000,000
Number of directors to whom the Company agreed to issue warrants | director		2									2
Principal repayment											$ 0	5,000,000	0
Interest expense			$ 5,000	$ 12,000	$ 13,000	$ 8,000	$ 0	$ 239,000	$ 362,000	$ 305,000	$ 38,000	906,000	385,000
Accretion of debt discount												$ 784,000	$ 347,000
Wells Fargo Term Loan | Credit Agreement
Short-term Debt [Line Items]
Credit agreement term		1 year
Principal amount		$ 5,000,000
Proceeds from borrowings		$ 5,000,000
Principal repayment	$ 5,000,000

Warrants - Notes Payable to Rel

Warrants - Notes Payable to Related Parties (Details) $ in Thousands	Dec. 31, 2016USD ($)Installment	Dec. 31, 2016USD ($)
Related Party Transactions
Ownership interest acquired	100.00%	100.00%
Majority Shareholder | Biohaven Pharmaceuticals, Inc.
Related Party Transactions
Ownership interest acquired	100.00%	100.00%
Aggregate purchase consideration | $	$ 595	$ 595
Number of installments in which notes are payable	5
Number of interest only related party notes payable installments	4
Interest rate of notes	4.50%

Warrants - Guarantor and Co-Gua

Warrants - Guarantor and Co-Guarantor Warrants (Details) $ in Thousands	Jan. 26, 2018USD ($)	Aug. 30, 2016director	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)director	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Guarantor And Co-Guarantor Warrants
Reclassification of fair value of warrant liability to shareholders' equity	$ 5,203										$ 5,203
Number of directors to whom the Company agreed to issue warrants | director		2									2
Fair value of warrant liability			$ 0				$ 4,021				$ 0	$ 4,021
Income (expense) from change in fair value of warrant liability			$ 0	$ 0	$ 0	$ 1,182	$ (2,268)	$ 2,426	$ 2,629	$ 454	$ 1,182	$ 3,241	$ (154)
Directors
Guarantor And Co-Guarantor Warrants
Number of directors to whom the Company agreed to issue warrants | director		2

Convertible Preferred Shares (D

Convertible Preferred Shares (Details) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended				12 Months Ended
	May 31, 2017	Feb. 28, 2017	Oct. 31, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Preferred Shares
Accretion of beneficial conversion feature on Series A preferred shares				$ 0	$ 0	$ 8,006	$ 4,000	$ 0	$ 12,006	$ 0
Conversion of Series A convertible preferred shares to common shares (shares)	9,358,560
AstraZeneca
Preferred Shares
Conversion of Series A convertible preferred shares to common shares (shares)	538,149
Series A Convertible Preferred Shares
Preferred Shares
Preferred stock issued and sold (shares)		4,305,182	4,305,209						4,305,182	4,305,209
Accretion of beneficial conversion feature on Series A preferred shares		$ 2,406							$ (12,006)
Conversion of Series A convertible preferred shares to common shares (shares)	9,358,560
Series A Convertible Preferred Shares | AstraZeneca
Preferred Shares
Preferred stock issued and sold (shares)			538,150

Common Shares (Details)

Common Shares (Details) $ / shares in Units, $ in Thousands	May 11, 2017shares	Jun. 30, 2018USD ($)shares	Jul. 31, 2016USD ($)$ / sharesshares	Feb. 29, 2016USD ($)$ / sharesshares	Dec. 31, 2018USD ($)voteshares	Dec. 31, 2017USD ($)shares	Dec. 31, 2016USD ($)shares	May 31, 2017$ / sharesshares	Apr. 21, 2017shares
Stockholders' Equity Note [Abstract]
Number of shares authorized (shares) | shares					200,000,000	200,000,000	38,000,000	200,000,000	50,000,000
Common stock, par value (in dollars per share) | $ / shares								$ 0
Number of votes | vote					1
Common stock sold (shares) | shares	1,883,523	1,111,111	1,090,500	429,000
Issuance price per share (in dollars per share) | $ / shares			$ 7.70	$ 7
Proceeds from issuance of common shares | $		$ 49,889	$ 8,299	$ 2,980	$ 190,125	$ 179,996	$ 11,399
Stock issuance costs | $		$ 111	$ 97	$ 23	$ 2,987	$ 5,068	$ 1,507

Common Shares - Other Info (Det

Common Shares - Other Info (Details) - USD ($) $ / shares in Units, $ in Thousands	May 11, 2017	May 09, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Apr. 30, 2018	Mar. 31, 2018	Jan. 31, 2018	May 31, 2017	Jul. 31, 2016	Feb. 29, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Common stock sold (shares)	1,883,523				1,111,111					1,090,500	429,000
Issuance price per share (in dollars per share)										$ 7.70	$ 7
Other offering expenses					$ 111					$ 97	$ 23	$ 2,987	$ 5,068	$ 1,507
Aggregate net proceeds from the IPO and underwriters issuance									$ 176,128
Common shares value issued												230,339	176,128	11,279
Proceeds from issuance of common shares					$ 49,889					$ 8,299	$ 2,980	190,125	$ 179,996	$ 11,399
Issuance of common shares as payment for license agreement												$ 4,080
BMS Agreement
Upfront payment under the BMS Amendment							$ 50,000
ALS Biopharma
Exercise of ALS Biopharma warrants, net of share settlement						261,140		228,219
Number of shares issued upon warrant exercise						325,000		275,000
IPO
Common stock sold (shares)		9,900,000							9,900,000
Issuance price per share (in dollars per share)		$ 17							$ 17
Proceeds from issuance of common shares upon completion of initial public offering, net of commissions and underwriting discounts		$ 152,651							$ 152,651
Underwriting discounts and commissions		11,781							11,781
Other offering expenses		$ 3,868							$ 3,868
Convertible preferred stock converted into shares of common stock		9,358,560
Over allotment option
Common stock sold (shares)	1,485,000								1,485,000
Proceeds from issuance of common shares upon completion of initial public offering, net of commissions and underwriting discounts	$ 23,478								$ 23,478
Underwriting discounts and commissions	1,767								$ 1,767
Aggregate net proceeds from the IPO and underwriters issuance	$ 176,128
Over allotment option | BMS and AstraZeneca
Common stock sold (shares)	1,883,523
Common shares value issued	$ 0
Private Placement
Common stock sold (shares)							2,000,000
Issuance price per share (in dollars per share)							$ 27.50
Net proceeds from underwriting							$ 52,000
Underwriting discounts and commissions							2,800
Other offering expenses							$ 200
Underwritten Public Offering
Common stock sold (shares)			3,859,060
Common stock sold, price per share (in dollars per share)			$ 37.25									$ 37.25
Proceeds from issuance of common shares			$ 134,485
Gross proceeds from sale of common shares												$ 143,750
License Agreement | AstraZeneca
Payment for license agreement				$ 3,000
Issuance of common shares as payment for license agreement				$ 4,080
Issuance of common shares as payment for license agreement (shares)				109,523

Share-Based Compensation - 2014

Share-Based Compensation - 2014 Equity Incentive Plan (Details) - 2014 Equity Incentive Plan - shares	12 Months Ended
	Dec. 31, 2018	Jan. 31, 2017	Dec. 31, 2016
Share-Based Compensation
Shares authorized		4,899,230	4,000,000
Minimum
Share-Based Compensation
Percentage of exercise price per share of stock options over fair market value of common share (percent)	100.00%
Maximum
Share-Based Compensation
Stock option term	10 years

Share-Based Compensation - 2017

Share-Based Compensation - 2017 Equity Incentive Plan and Employee Share Purchase Plan (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Jan. 31, 2019	Jan. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Apr. 30, 2017
Share-Based Compensation
Share-based compensation expense			$ 16,925	$ 13,239	$ 4,603
2017 Equity Incentive Plan
Share-Based Compensation
Shares authorized						2,713,113
Number of shares remained available for future grant			681,307	1,138,903
Number of additional shares to be issued		1,437,227
2017 Equity Incentive Plan | Employees and directors
Share-Based Compensation
Stock option granted			1,810,000	2,335,106	417,875
Share-based compensation expense			$ 11,246	$ 5,210	$ 2,284
2017 Equity Incentive Plan | Non-employees
Share-Based Compensation
Stock option granted			145,000	273,537	199,050
Share-based compensation expense			$ 5,679	$ 8,029	$ 2,319
2017 Equity Incentive Plan | Maximum
Share-Based Compensation
Contractual term (in years)			10 years
Subsequent Event | 2017 Equity Incentive Plan
Share-Based Compensation
Number of additional shares to be issued	1,767,901

Share-Based Compensation - Stoc

Share-Based Compensation - Stock Option Valuation (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock Option Valuation
Fair value of common share	$ 22	$ 12.31	$ 4.09
Plans
Stock Option Valuation
Exercise price	$ 32.35
Plans | Employees and directors
Stock Option Valuation
Risk-free interest rate	2.91%	2.10%	2.19%
Expected term (in years)	6 years 3 months	6 years 7 days	5 years 9 months
Expected volatility	73.03%	73.26%	70.58%
Expected dividend yield	0.00%	0.00%	0.00%
Exercise price	$ 32.35	$ 18.47	$ 9.29
Plans | Non-employees
Stock Option Valuation
Risk-free interest rate	3.06%	2.35%	2.54%
Expected term (in years)	10 years	10 years	10 years
Expected volatility	74.50%	71.12%	67.16%
Expected dividend yield	0.00%	0.00%	0.00%
Exercise price	$ 32.42	$ 18.23	$ 9.29

Share-Based Compensation - St_2

Share-Based Compensation - Stock Options (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Additional disclosure
Weighted average grant-date fair value per share	$ 22	$ 12.31	$ 4.09
Total fair value of options vested	$ 25,876	$ 15,494	$ 3,381
Plans
Number of shares
Outstanding at the beginning	6,151,643
Granted	1,955,000
Exercised	(577,288)
Forfeited	(85,176)
Outstanding at the end	7,444,179	6,151,643
Options exercisable as of December 31,2018	3,854,598
Options unvested as of December 31,2018	3,589,581
Weighted Average Exercise Price
Outstanding at the beginning (in dollars per share)	$ 9.97
Granted (in dollars per share)	32.35
Exercised (in dollars per share)	6.26
Forfeited (in dollars per share)	22.54
Outstanding at the end (in dollars per share)	15.99	$ 9.97
Options exercisable as of December 31,2018	6.26
Options unvested as of December 31,2018	$ 26.46
Additional disclosure
Outstanding average remaining contractual term (in years)	8 years 18 days	8 years 5 months 1 day
Granted weighted average remaining contractual term (in years)	9 years 8 months 12 days
Options exercisable as of December 31,2018	6 years 11 months 1 day
Options unvested as of December 31,2018 (in years)	9 years 3 months 3 days
Outstanding aggregate intrinsic value	$ 156,518	$ 107,072	$ 15,991
Options exercisable as of December 31,2018 (in dollars)	118,452
Options unvested as of December 31,2018 (in dollars)	$ 38,066

Share-Based Compensation (Detai

Share-Based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-Based Compensation
Share-based compensation expense	$ 16,925	$ 13,239	$ 4,603
Total unrecognized compensation cost	$ 61,583
Weighted average period of unrecognized compensation cost	3 years 3 months 18 days
Research and development expenses
Share-Based Compensation
Share-based compensation expense	$ 8,371	6,933	2,382
General and administrative expenses
Share-Based Compensation
Share-based compensation expense	$ 8,554	$ 6,306	$ 2,221

License and Other Agreements -

License and Other Agreements - Yale Agreement (Details)	May 11, 2017shares	Jun. 30, 2018shares	Jul. 31, 2016shares	Feb. 29, 2016shares	Sep. 30, 2013USD ($)shares	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Oct. 31, 2017USD ($)
License agreements
Common stock sold (shares) | shares	1,883,523	1,111,111	1,090,500	429,000
Yale Agreement | Collaborative arrangement
License agreements
Common stock sold (shares) | shares					250,000
Right to purchase securities in specified future equity offering (as a percent)					0.10
Fair value of obligation					$ 0	$ 0	$ 0		$ 0
Percentage of dollar value of all initial and future consideration					0.05
Consideration payable upon change-of-control event					$ 1,500,000
Fair value of derivative liability					14,000
Other income (expense) for change in the fair value of the derivative liability						0	512,000	$ (65,000)
Fair value of derivative liability					0	$ 0	$ 0		$ 0
Minimum milestone payment to be paid					$ 1,000,000
Notice period to Yale to terminate the Yale Agreement					90 days
Last patent right expiration period					10 years
Yale Agreement | Collaborative arrangement | Maximum
License agreements
Milestone payment to be paid					$ 2,000,000
Extension of due diligence requirements					1 year
Amount of payment subject to due diligence extension					$ 150,000

License and Other Agreements _2

License and Other Agreements - MGH Agreement (Details) - MGH Agreement - Collaborative arrangement - Maximum $ in Thousands	1 Months Ended
	Sep. 30, 2014USD ($)
License agreements
Milestone payment to be paid	$ 750
Commercial milestone payment to be paid	$ 2,500
Extension of due diligence requirements	1 year
Amount of payment subject to due diligence extension	$ 300

License and Other Agreements _3

License and Other Agreements - ALS Biopharma Agreement (Details) $ / shares in Units, $ in Thousands	May 11, 2017shares	Jun. 30, 2018shares	Jul. 31, 2016shares	Feb. 29, 2016shares	Aug. 31, 2015USD ($)claimprodrug$ / sharesshares	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 ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
License agreements
Common stock sold (shares) | shares	1,883,523	1,111,111	1,090,500	429,000
Research and development | $						$ 37,958	$ 47,362	$ 29,052	$ 75,579	$ 22,686	$ 34,996	$ 21,019	$ 10,740	$ 189,951	$ 89,441	$ 55,529
ALS Biopharma Agreement | Product related collaborative arrangements
License agreements
Number of prodrugs of glutamate modulating agents | prodrug					300
Milestone payment to be paid | $					$ 3,000
Milestone payment to be paid for subsequently developed products | $					$ 1,000
Common stock sold (shares) | shares					50,000
Number of claims | claim					1
Research and development | $														$ 0	$ 0	$ 3,127
Common stock warrants issued, immediately exercisable | ALS Biopharma Agreement | Product related collaborative arrangements
License agreements
Common stock sold (shares) | shares					275,000
Common stock sold, price per share (in dollars per share) | $ / shares					$ 5.60
Common stock warrants issued, exercisable upon filing drug application | ALS Biopharma Agreement | Product related collaborative arrangements
License agreements
Common stock sold (shares) | shares					325,000
Common stock sold, price per share (in dollars per share) | $ / shares					$ 5.60

License and Other Agreements _4

License and Other Agreements - Rutgers Agreement (Details) - Rutgers Agreement - Collaborative arrangement $ in Thousands	1 Months Ended
	Jun. 30, 2016USD ($)
License agreements
Milestone payment to be paid	$ 825
Change of control fee	0.0030
Fair value of derivative liability	$ 0
Last patent right expiration period	10 years
Maximum
License agreements
Extension of due diligence requirements	1 year
Amount of payment subject to due diligence extension	$ 500
Minimum
License agreements
Guaranteed royalties	100
Consideration payable upon change-of-control event	$ 100

License and Other Agreements _5

License and Other Agreements - BMS Agreement (Details) $ / shares in Units, $ in Thousands	May 11, 2017shares	Jun. 30, 2018shares	Mar. 31, 2018USD ($)	May 31, 2017shares	Jul. 31, 2016USD ($)product$ / sharesshares	Feb. 29, 2016$ / sharesshares	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 ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
License agreements
Net proceeds															$ 0	$ 40,000	$ 40,000
Common shares value issued															230,339	176,128	11,279
Fair value of common share | $ / shares					$ 7.70	$ 7
Fair value of contingent equity liability											$ 0	$ 0	$ 9,707	$ 3,375	0	13,082	2,263
Common stock sold (shares) | shares	1,883,523	1,111,111			1,090,500	429,000
Change in fair value of contingent equity liability															0	13,082	2,263
Research and development							$ 37,958	$ 47,362	$ 29,052	$ 75,579	$ 22,686	$ 34,996	$ 21,019	$ 10,740	189,951	89,441	55,529
BMS Agreement
License agreements
Upfront payment under the BMS Amendment			$ 50,000
BMS Agreement | Product related collaborative arrangements
License agreements
Common shares value issued					$ 12,500
Fair value of common share | $ / shares					$ 9.2911
Common stock sold (shares) | shares				1,345,374
Change in fair value of contingent equity liability																8,809	$ 13,125
Research and development															$ 2,000	$ 5,000
BMS Agreement | Product related collaborative arrangements | Maximum
License agreements
Milestone payment to be paid					$ 127,500
Milestone payment to be paid for any product other than rimegepant					74,500
Commercial milestone payment to be paid					$ 150,000
Right to purchase shares fully diluted (as a percent)					0.08
BMS Agreement | Product related collaborative arrangements | Minimum
License agreements
Number of licensed products | product					1
Net proceeds					$ 30,000
Minimum investment in equity					22,000
BMS Agreement | Research and development expenses | Product related collaborative arrangements
License agreements
Fair value of contingent equity liability					$ 13,125

License and Other Agreements _6

License and Other Agreements - AstraZeneca Agreement (Details) $ / shares in Units, $ in Thousands	May 11, 2017shares	Sep. 30, 2018USD ($)shares	Jun. 30, 2018shares	May 31, 2017shares	Oct. 31, 2016USD ($)$ / sharesshares	Jul. 31, 2016$ / sharesshares	Feb. 29, 2016$ / sharesshares	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 ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
License agreements
Issuance of common shares as payment for license agreement																$ 4,080
Common shares value issued																230,339	$ 176,128	$ 11,279
Fair value of common share | $ / shares						$ 7.70	$ 7
Common stock sold (shares) | shares	1,883,523		1,111,111			1,090,500	429,000
Change in fair value of contingent equity liability																0	13,082	2,263
Research and development								$ 37,958	$ 47,362	$ 29,052	$ 75,579	$ 22,686	$ 34,996	$ 21,019	$ 10,740	$ 189,951	89,441	55,529
AstraZeneca | Product related collaborative arrangements
License agreements
Expiration period based on country by country basis after first commercial sale (in years)					10 years
Common stock value agreed to issue					$ 10,000
Common shares value issued					$ 5,000
Percentage of shares to be issued upon closing of the transaction					0.50
Percentage of remaining shares to be issued after contractual events					0.50
Fair value of common share | $ / shares					$ 9.2911
Preferred stock issued and sold (shares) | shares					538,150
Aggregate fair value					$ 5,000
Reclassification of contingent equity to the carrying value					5,000
Common stock sold (shares) | shares				538,149
Change in fair value of contingent equity liability																	$ 4,273	$ 938
AstraZeneca | Product related collaborative arrangements | Maximum
License agreements
Milestone payment to be paid					30,000
Milestone payment to be paid for any product other than Rett syndrome					60,000
Commercial milestone payment to be paid					$ 120,000
Right to purchase shares fully diluted (as a percent)					0.08
AstraZeneca | Product related collaborative arrangements | Minimum
License agreements
Minimum investment in equity					$ 30,000
License Agreement | AstraZeneca
License agreements
Payment for license agreement		$ 3,000
Issuance of common shares as payment for license agreement (shares) | shares		109,523
Issuance of common shares as payment for license agreement		$ 4,080
Milestone payment due upon achievement of specified regulatory and commercial milestones		55,000
Milestone payment due upon achievement of specified sales-based milestones		$ 50,000
Agreement time period		10 years

License and Other Agreements _7

License and Other Agreements - RPharm Agreement (Details) - USD ($) $ in Thousands

1 Months Ended

3 Months Ended

12 Months Ended

Nov. 30, 2017

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

License agreements

Research and development

$ 37,958

$ 47,362

$ 29,052

$ 75,579

$ 22,686

$ 34,996

$ 21,019

$ 10,740

$ 189,951

$ 89,441

$ 55,529

RPharm Agreement | Collaborative arrangement

License agreements

Research and development

$ 1,400

RPharm Agreement | Collaborative arrangement | Maximum

License agreements

Commercial milestone payment to be paid

$ 2,500

License and Other Agreements _8

License and Other Agreements - Catalent Agreement for Rimegepant (Details) - Collaborative arrangement - Catalent Agreement $ in Thousands	1 Months Ended
	Jan. 31, 2018USD ($)
License agreements
Last patent right expiration period	10 years
Automatic extension period	1 year
Maximum
License agreements
Milestone payment to be paid	$ 1,500

License and Other Agreements _9

License and Other Agreements - Revenue Participation Rights with RPI Finance Trust (Details) - RPI Funding Agreement $ in Thousands	1 Months Ended
	Jun. 30, 2018USD ($)
License agreements
Proceeds from royalty agreement	$ 100,000
Percentage of revenue participation rights available for repurchase (percent)	100.00%
Price of revenue participation rights available for repurchase	$ 155,000
Revenue Participation Right, tranche one
License agreements
Participation rate for revenue participation right (percent)	2.10%
Revenue Participation Right, tranche one | Maximum
License agreements
Annual global net sales threshold for revenue participation right	$ 1,500,000
Revenue Participation Right, tranche two
License agreements
Participation rate for revenue participation right (percent)	1.50%

Income Taxes - General Informat

Income Taxes - General Information (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 [Abstract]
Provision for income taxes	$ 194	$ 161	$ 25	$ 87	$ 359	$ 55	$ 399	$ 193	$ 467	$ 1,006	$ 90
Income (loss) before provision for income taxes
BVI									(246,829)	(130,359)	(63,677)
Foreign (U.S)									6,374	4,175	233
Loss before provision for income taxes	$ (54,556)	$ (61,280)	$ (39,244)	$ (85,375)	$ (26,723)	$ (42,860)	$ (38,041)	$ (18,560)	$ (240,455)	$ (126,184)	$ (63,444)

Income Taxes - Provision for In

Income Taxes - 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
Current income tax provision:
BVI									$ 0	$ 0	$ 0
Foreign (U.S. federal and state)									467	997	99
Total current income tax provision									467	997	99
Deferred income tax provision (benefit):
BVI									0	0	0
Foreign (U.S. federal and state)									0	9	(9)
Total deferred income tax provision (benefit)									0	9	(9)
Total provision for income taxes	$ 194	$ 161	$ 25	$ 87	$ 359	$ 55	$ 399	$ 193	$ 467	$ 1,006	$ 90
Reconciliation of BVI statutory income tax rate
BVI statutory income tax rate (as a percent)									0.00%	0.00%	0.00%
Foreign tax rate differential									0.60%	1.20%	0.10%
Tax Credits									(3.50%)	(2.70%)	0.00%
Change in valuation allowance									3.40%	2.20%	0.00%
Other									(0.30%)	0.10%	0.00%
Effective income tax rate									0.20%	0.80%	0.10%

Income Taxes - Net Deferred Tax

Income Taxes - Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Foreign net operating loss carryforwards	$ 0	$ 0
Tax credits	11,396	2,790
Other	1	1
Total deferred tax assets	11,397	2,791
Deferred tax liabilities:
Other	(440)	(7)
Total deferred tax liabilities	(440)	(7)
Valuation allowance	(10,957)	(2,784)	$ 0	$ (16)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Tax Credit Carry

Income Taxes - Tax Credit Carryforwards and Tax Reforms (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Orphan Drug Credit
Tax Credit Carryforward [Line Items]
Tax credit carryforwards		$ 2,544
Orphan Drug Credit | Maximum
Tax Credit Carryforward [Line Items]
Limitation on reduction of Orphan Drug Credit	50.00%
Orphan Drug Credit | Minimum
Tax Credit Carryforward [Line Items]
Limitation on reduction of Orphan Drug Credit	25.00%
Federal and State | Research and Development
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	$ 1,623	$ 7,569

Income Taxes - Valuation allowa

Income Taxes - Valuation allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Valuation allowance as of beginning of year	$ 2,784	$ 0	$ 16
Decreases recorded as benefit to income tax provision	0	0	16
Increases recorded to income tax provision	8,173	2,784	0
Valuation allowance as of end of year	$ 10,957	$ 2,784	$ 0

Income Taxes - Unrecognized Tax

Income Taxes - Unrecognized Tax Benefits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Unrecognized tax benefits
Accrued interest or penalties related to uncertain tax positions	$ 0
Income tax penalties and interest expense	$ 0	$ 0

Net Loss per Share (Details)

Net Loss per 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					$ (27,082)	$ (42,915)	$ (38,440)	$ (18,753)	$ (240,922)	$ (127,190)	$ (63,534)
Net (income) loss attributable to non-controlling interests									0	0	(143)
Accretion of beneficial conversion feature on Series A preferred shares					0	0	(8,006)	(4,000)	0	(12,006)	0
Net loss attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd.	$ (54,750)	$ (61,441)	$ (39,269)	$ (85,462)	$ (27,082)	$ (42,915)	$ (46,446)	$ (22,753)	$ (240,922)	$ (139,196)	$ (63,677)
Denominator:
Weighted average common shares outstanding - basic and diluted	40,938,709	40,147,735	38,942,545	36,793,090	35,984,111	35,930,698	26,038,192	13,088,861	39,188,458	27,845,576	12,608,366
Net loss per share attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd. - basic and diluted	$ (1.34)	$ (1.53)	$ (1.01)	$ (2.32)	$ (0.75)	$ (1.19)	$ (1.78)	$ (1.74)	$ (6.15)	$ (5)	$ (5.05)

Net Loss per Share - Antidiluti

Net Loss per Share - Antidilutive Securities Excluded from EPS (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Securities excluded from computation of diluted net loss per share
Anti-dilutive securities excluded from calculation of diluted net loss per share	7,665,930	6,973,394	4,464,425
Options to purchase common shares
Securities excluded from computation of diluted net loss per share
Anti-dilutive securities excluded from calculation of diluted net loss per share	7,444,179	6,151,643	3,864,425
Warrants to purchase common shares
Securities excluded from computation of diluted net loss per share
Anti-dilutive securities excluded from calculation of diluted net loss per share	221,751	821,751	600,000

Net Loss per Share - Warrants a

Net Loss per Share - Warrants and Contingent Equity Liability (Details) - $ / shares	May 11, 2017	May 09, 2017	Jun. 30, 2018	May 31, 2017	Jul. 31, 2016	Feb. 29, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 31, 2017
Contingent Equity Liability
Common stock sold (shares)	1,883,523		1,111,111		1,090,500	429,000
BMS Agreement | Product related collaborative arrangements
Contingent Equity Liability
Common stock sold (shares)				1,345,374
AstraZeneca | Product related collaborative arrangements
Contingent Equity Liability
Common stock sold (shares)				538,149
IPO
Contingent Equity Liability
Common stock sold (shares)		9,900,000		9,900,000
Common Shares
Contingent Equity Liability
Common stock sold (shares)							6,970,171	11,385,000	1,519,500
Common Shares | Common stock warrants issued in connection with the Credit Agreement | Credit Agreement
Warrants
Exercise price (in dollars per share)										$ 9.2911
Common Shares | Common stock warrants issued to Guarantor in connection with the Credit Agreement | Credit Agreement
Warrants
Shares that warrant can purchase										107,500
Common Shares | Common stock warrants issued to Co-Guarantor in connection with the Credit Agreement | Credit Agreement
Warrants
Shares that warrant can purchase										107,500

Commitments and Contingencies -

Commitments and Contingencies - Lease Agreements and License Agreements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Oct. 31, 2018	Aug. 31, 2017
Leased and License Agreements
Accrued clinical trial costs	$ 6,753	$ 3,582
Office Space, New Haven, CT
Leased and License Agreements
Rent expense	114	73	$ 0
New Headquarters, New Haven, CT
Leased and License Agreements
Rent expense	43	75	0
Capitalized construction costs	$ 3,404	$ 2,198	$ 0
Lease agreement, term (in months)					85 months
Renewal term (in months)					120 months
BMS Agreement
Leased and License Agreements
Accrued clinical trial costs				$ 2,000

Commitments and Contingencies_2

Commitments and Contingencies - Research Commitments (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
Contingencies and commitments
Minimum payments under CRO preclinical studies and clinical trials	$ 37,958	$ 47,362	$ 29,052	$ 75,579	$ 22,686	$ 34,996	$ 21,019	$ 10,740	$ 189,951	$ 89,441	$ 55,529
Research Commitments with CROs | Minimum
Contingencies and commitments
Minimum payments under CRO preclinical studies and clinical trials									$ 16,949

Related Party Transactions (Det

Related Party Transactions (Details) $ / shares in Units, $ in Thousands	Sep. 30, 2018USD ($)shares	Jan. 26, 2017$ / sharesshares	Dec. 31, 2016USD ($)	Sep. 30, 2013investor	Nov. 30, 2018USD ($)shares	Jan. 31, 2018USD ($)shares	Oct. 31, 2017USD ($)shares	May 31, 2017USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)shares	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)founder
Related Party Transactions
Common stock value									$ 554,384				$ 311,061				$ 554,384	$ 311,061
Purchase of Kleo common stock																	6,375	6,627	$ 3,000
Number of founders | founder																			3
Equity share percentage owned by each founders			33.33%																33.33%
Ownership interest acquired			100.00%																100.00%
Research and development									37,958	$ 47,362	$ 29,052	$ 75,579	22,686	$ 34,996	$ 21,019	$ 10,740	189,951	89,441	$ 55,529
Chief Executive Officer | License Agreement with Yale
Related Party Transactions
Number of inventors of the patents licensed from Yale | investor				1
Common shares issued | shares	250,000
Percent of Company's outstanding equity fully diluted	5.10%
Common stock value	$ 152									$ 152
Recognized research and development expenses																	0	0	$ 0
Payments to related party									$ 0			$ 600	$ 0				$ 0	0
Shareholders and members of the Board of Directors | Guarantor and Co-Guarantor Warrants | Guarantor and Co-Guarantor Warrants
Related Party Transactions
Common shares issued | shares		107,500
Exercise price of warrants (per share) | $ / shares		$ 9.2911
Equity Investment | Kleo Pharmaceuticals, Inc.
Related Party Transactions
Shares purchased in equal tranches | shares						1,375,000
Purchase of Kleo common stock						$ 1,375
Ownership percentage									41.90%								41.90%
Majority Shareholder | Biohaven Pharmaceuticals, Inc.
Related Party Transactions
Number of founders | founder																			3
Ownership interest acquired			100.00%																100.00%
Aggregate purchase consideration			$ 595																$ 595
Repayment of accrued interest amount to related party								$ 9
IPO | Biohaven Pharmaceuticals, Inc.
Related Party Transactions
Repayment of principal amount to related party																		595
Kleo
Related Party Transactions
Shares of Kleo common stock purchased | shares					1,420,818		2,049,543
Purchase of Kleo common stock					$ 5,000		$ 2,253										$ 6,375	$ 6,979
Shares of Kleo common stock purchased from Chief Executive Officer | shares																500,000
Ownership percentage						46.60%			41.90%				43.30%				41.90%	43.30%

Acquisition of Biohaven Pharm_2

Acquisition of Biohaven Pharmaceuticals, Inc (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)founder
Acquisition of Biohaven Pharmaceuticals, Inc.
Number of founders | founder			3
Equity share percentage owned by each founders			33.33%
Ownership interest acquired			100.00%
Net loss attributable to non-controlling interests	$ 0	$ 0	$ (143)
Biohaven Pharmaceuticals, Inc. | Majority Shareholder
Acquisition of Biohaven Pharmaceuticals, Inc.
Number of founders | founder			3
Equity share percentage owned by each founders			33.00%
Ownership interest acquired			100.00%
Aggregate purchase consideration			$ 595
Carrying value of non-controlling interest derecognized			86
Reduction to additional paid-in capital			509
Net loss attributable to non-controlling interests			$ (143)

Quarterly Financial Data (Una_3

Quarterly Financial Data (Unaudited) (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
Operating expenses:
Research and development	$ 37,958	$ 47,362	$ 29,052	$ 75,579	$ 22,686	$ 34,996	$ 21,019	$ 10,740	$ 189,951	$ 89,441	$ 55,529
General and administrative	10,108	7,574	9,064	7,857	5,614	4,571	4,199	3,757	34,603	18,141	5,109
Total operating expenses	48,066	54,936	38,116	83,436	28,300	39,567	25,218	14,497	224,554	107,582	60,638
Loss from operations	(48,066)	(54,936)	(38,116)	(83,436)	(28,300)	(39,567)	(25,218)	(14,497)	(224,554)	(107,582)	(60,638)
Other income (expense):
Interest expense	(5)	(12)	(13)	(8)	0	(239)	(362)	(305)	(38)	(906)	(385)
Non-cash interest expense on liability related to sale of future royalties	(5,592)	(5,633)	(501)	0					(11,726)	0	0
Interest income					(10)	10	0	0
Change in fair value of warrant liability	0	0	0	(1,182)	2,268	(2,426)	(2,629)	(454)	(1,182)	(3,241)	154
Change in fair value of derivative liability					0	0	223	289	0	512	(65)
Change in fair value of contingent equity liability					0	0	(9,707)	(3,375)	0	(13,082)	(2,263)
Loss from equity method investment	(742)	(697)	(641)	(728)	(681)	(638)	(348)	(218)	(2,808)	(1,885)	(247)
Other	(151)	(2)	27	(21)					(147)	0	0
Total other income (expense), net	(6,490)	(6,344)	(1,128)	(1,939)	1,577	(3,293)	(12,823)	(4,063)	(15,901)	(18,602)	(2,806)
Loss before provision for income taxes	(54,556)	(61,280)	(39,244)	(85,375)	(26,723)	(42,860)	(38,041)	(18,560)	(240,455)	(126,184)	(63,444)
Provision for income taxes	194	161	25	87	359	55	399	193	467	1,006	90
Net loss and comprehensive loss					(27,082)	(42,915)	(38,440)	(18,753)	(240,922)	(127,190)	(63,534)
Accretion of beneficial conversion feature on Series A preferred shares					0	0	(8,006)	(4,000)	0	(12,006)	0
Net loss attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd.	$ (54,750)	$ (61,441)	$ (39,269)	$ (85,462)	$ (27,082)	$ (42,915)	$ (46,446)	$ (22,753)	$ (240,922)	$ (139,196)	$ (63,677)
Net loss per share attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd. - basic and diluted	$ (1.34)	$ (1.53)	$ (1.01)	$ (2.32)	$ (0.75)	$ (1.19)	$ (1.78)	$ (1.74)	$ (6.15)	$ (5)	$ (5.05)
Weighted average common shares outstanding - basic and diluted	40,938,709	40,147,735	38,942,545	36,793,090	35,984,111	35,930,698	26,038,192	13,088,861	39,188,458	27,845,576	12,608,366
Share-based compensation expense									$ 16,925	$ 13,239	$ 4,603
Out-of-period adjustment
Other income (expense):
Share-based compensation expense					$ 3,294