Summary of Significant Accounting Policies - Summary of Impact of Adopting ASC 606 on Unaudited Condensed Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue		$ 7,254	$ 22,254	$ 18,701
Loss from operations	$ (19,507)	(6,653)	(37,057)	(18,144)
Net loss	$ (18,849)	(6,071)	(35,130)	(16,852)
Collaboration Revenue
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue		$ 7,254	22,254	$ 18,701
Adjustments | ASC 2014-09
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue			8,791
Loss from operations			8,791
Net loss			8,791
Adjustments | Collaboration Revenue | ASC 2014-09
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue			8,791
Balances Without the Adoption of ASC 606 | ASC 2014-09
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue			31,045
Loss from operations			(28,266)
Net loss			(26,339)
Balances Without the Adoption of ASC 606 | Collaboration Revenue | ASC 2014-09
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue			$ 31,045