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				6 Months Ended		12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Collaboration revenue	$ 392		$ 1,395		$ 662	$ 2,524	$ 5,322
Loss from operations	(27,973)		(13,422)		(53,096)	(24,119)	(59,926)
Net loss	$ (26,270)	$ (23,852)	$ (12,426)	$ (10,217)	$ (50,122)	$ (22,643)	$ (55,577)
Net loss per share attributable to common stockholders-basic and diluted	$ (0.61)		$ (0.34)		$ (1.25)	$ (1.15)	$ (1.95)
ASC 606 | As Originally Reported Under ASC 605
Collaboration revenue			$ 926			$ 1,749	$ 3,684
Loss from operations			(13,891)			(24,894)	(61,564)
Net loss			$ (12,895)			$ (23,418)	$ (57,215)
Net loss per share attributable to common stockholders-basic and diluted			$ (0.35)			$ (1.19)	$ (2.00)
ASC 606 | Effect of Change
Collaboration revenue			$ 469			$ 775	$ 1,638
Loss from operations			469			775	1,638
Net loss			$ 469			$ 775	$ 1,638
Net loss per share attributable to common stockholders-basic and diluted			$ 0.01			$ 0.04	$ 0.05