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Basis of Presentation and Significant Accounting Policies (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current	$ 7,377		$ 7,377		$ 7,742	$ 7,202
Deferred revenue, net of current portion	14,134		14,134		19,576	13,637
Accumulated deficit	445,627		445,627		318,819	312,340
Revenues	20,798	$ 1,663	44,327	$ 5,383
Net loss	$ (34,029)	$ (47,043)	$ (126,809)	$ (125,353)
Basic and diluted net loss per common share (in usd per share)	$ (0.81)	$ (1.28)	$ (3.13)	$ (3.50)
Change in Contract with Customer, Liability [Roll Forward] [Roll Forward]
Beginning balance			$ 27,318
Additions			500
Deductions			(6,307)
Ending balance	$ 21,511		21,511
Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current	6,348		6,348			7,202
Deferred revenue, net of current portion	9,090		9,090			13,637
Accumulated deficit	439,554		439,554			$ 312,340
Net loss	$ (34,164)		$ (127,214)
Basic and diluted net loss per common share (in usd per share)	$ (0.81)		$ (3.14)
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current	$ 1,029		$ 1,029		540
Deferred revenue, net of current portion	5,044		5,044		5,939
Accumulated deficit	(6,073)		(6,073)		$ 6,479
Net loss	$ (135)		$ (405)
Basic and diluted net loss per common share (in usd per share)	$ 0		$ 0.01
Revenue From Collaborative Agreements
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 20,617	$ 1,076	$ 43,670	$ 3,435
Revenue From Collaborative Agreements | Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	20,482		43,265
Revenue From Collaborative Agreements | Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 135		$ 405

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