Summary of Significant Accounting Policies - Impact of Adoption of ASC 606 on Previously Reported Amounts of Condensed Consolidated Statements of Operations and Comprehensive Loss (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Collaboration revenue	$ 441			$ 1,602			$ 1,103	$ 4,126	$ 5,322
Loss from operations	(31,288)			(15,635)			(84,384)	(39,754)	(59,926)
Net loss	$ (29,627)	$ (26,270)	$ (23,852)	$ (14,148)	$ (12,426)	$ (10,217)	$ (79,749)	$ (36,791)	$ (55,577)
Net loss per share attributable to common stockholders-basic and diluted	$ (0.67)			$ (0.38)			$ (1.92)	$ (1.42)	$ (1.95)
ASC 606 | As Originally Reported Under ASC 605
Collaboration revenue				$ 954				$ 2,704	$ 3,684
Loss from operations				(16,283)				(41,176)	(61,564)
Net loss				$ (14,796)				$ (38,213)	$ (57,215)
Net loss per share attributable to common stockholders-basic and diluted				$ (0.40)				$ (1.48)	$ (2.00)
ASC 606 | Effect of Change
Collaboration revenue				$ 648				$ 1,422	$ 1,638
Loss from operations				648				1,422	1,638
Net loss				$ 648				$ 1,422	$ 1,638
Net loss per share attributable to common stockholders-basic and diluted				$ 0.02				$ 0.06	$ 0.05