Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Revenue Recognition (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current—related party	$ 55,157	$ 51,142
Deferred revenue, net of current portion—related party	42,715	65,018
Total liabilities	110,323	129,551
Accumulated deficit	(163,907)	(89,615)
Total stockholders’ equity	104,129	167,109	$ (69,088)
Collaboration revenue—related party	65,201	71,644
Net loss	$ (27,379)	$ (16,443)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.84)	$ (0.57)
Calculated under Revenue Guidance in Effect before Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current—related party	$ 42,174
Deferred revenue, net of current portion—related party	22,844
Total liabilities	77,469
Accumulated deficit	(131,053)
Total stockholders’ equity	136,983
Collaboration revenue—related party	51,142	$ 71,600
Net loss	$ (41,438)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (1.27)
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current—related party	$ 12,983
Deferred revenue, net of current portion—related party	19,871
Total liabilities	32,854
Accumulated deficit	(32,854)	$ (46,900)
Total stockholders’ equity	(32,854)
Collaboration revenue—related party	14,059
Net loss	$ 14,059
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ 0.43