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		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Collaboration revenue	$ 270	$ 1,129	$ 5,322
Loss from operations	(25,123)	(10,696)	(59,926)
Net loss	$ (23,852)	$ (10,217)	$ (55,577)
Net loss per share attributable to common stockholders-basic and diluted	$ (0.64)	$ (4.09)	$ (1.95)
ASC 606 | As Originally Reported Under ASC 605
Collaboration revenue		$ 823	$ 3,684
Loss from operations		(11,002)	(61,564)
Net loss		$ (10,523)	$ (57,215)
Net loss per share attributable to common stockholders-basic and diluted		$ (4.21)	$ (2.00)
ASC 606 | Effect of Change
Collaboration revenue		$ 306	$ 1,638
Loss from operations		306	1,638
Net loss		$ 306	$ 1,638
Net loss per share attributable to common stockholders-basic and diluted		$ 0.12	$ 0.05