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Summary of Significant Accounting Policies - Adoption of 606 (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	Jan. 01, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current	$ 21,721				$ 7,202				$ 21,721	$ 7,202		$ 7,742
Deferred revenue, net of current portion	19,001				13,637				19,001	13,637		19,576
Accumulated deficit	(490,271)				(312,340)				(490,271)	(312,340)		(318,818)
Revenues	15,794	$ 20,798	$ 18,834	$ 4,695	152,359	$ 1,663	$ 1,666	$ 2,055	60,121	157,742	$ 91,880
Net loss	(44,644)	$ (34,029)	$ (43,244)	$ (49,536)	$ 105,727	$ (47,043)	$ (40,654)	$ (37,655)	$ (171,453)	$ (19,626)	$ (58,528)
Basic and diluted net loss per common share (in usd per share)									$ (4.19)	$ (0.54)	$ (1.69)
Pre-Adoption
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current	22,210								$ 22,210			7,202
Deferred revenue, net of current portion	12,573								12,573			13,637
Accumulated deficit	(484,332)								(484,332)			(312,340)
Net loss									$ (171,993)
Basic and diluted net loss per common share (in usd per share)									$ (4.20)
Effect of Change Higher/(Lower)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current	(489)								$ (489)
Deferred revenue, net of current portion	6,428								6,428
Accumulated deficit	$ (5,939)								(5,939)
Net loss									$ (540)
Basic and diluted net loss per common share (in usd per share)									$ 0.01
Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current												540
Deferred revenue, net of current portion												5,939
Accumulated deficit												$ (6,478)
Revenue from collaborative agreements
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									$ 58,644	$ 155,516	$ 86,582
Revenue from collaborative agreements | Pre-Adoption
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									58,104
Revenue from collaborative agreements | Effect of Change Higher/(Lower)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									$ 540

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