Basis of Presentation and Significant Accounting Policies (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current	$ 7,999		$ 7,999		$ 7,742	$ 7,202
Deferred revenue, net of current portion	15,948		15,948		19,576	13,637
Accumulated deficit	411,599		411,599		318,819	312,340
Revenues	18,834	$ 1,666	23,529	$ 3,720
Net loss	$ (43,244)	$ (40,654)	$ (92,780)	$ (78,310)
Basic and diluted net loss per common share (in usd per share)	$ (1.03)	$ (1.14)	$ (2.35)	$ (2.21)
Change in Contract with Customer, Liability [Roll Forward] [Roll Forward]
Beginning balance			$ 27,318
Additions			500
Deductions			(3,871)
Ending balance	$ 23,947		23,947
Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current	7,133		7,133			7,202
Deferred revenue, net of current portion	10,606		10,606			13,637
Accumulated deficit	405,391		405,391			$ 312,340
Net loss	$ (43,380)		$ (93,050)
Basic and diluted net loss per common share (in usd per share)	$ (1.03)		$ (2.35)
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	$ 866		$ 866		540
Deferred revenue, net of current portion	5,342		5,342		5,939
Accumulated deficit	(6,208)		(6,208)		$ 6,479
Net loss	$ (136)		$ (270)
Basic and diluted net loss per common share (in usd per share)	$ 0.00		$ 0.00
Revenue From Collaborative Agreements
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 18,552	$ 1,081	$ 23,053	$ 2,359
Revenue From Collaborative Agreements | Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	18,416		22,783
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	$ 136		$ 270