Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue - related party	$ 439	$ 344	$ 143	$ 14,434	$ 9,764	$ 1,730	$ 2,428	$ 45,495	$ 15,360	$ 59,417	$ 12,826
Type of Revenue [Extensible List]	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember
Total operating expenses	$ 16,771	$ 17,900	$ 18,653	$ 19,402	$ 15,345	$ 19,760	$ 19,011	$ 14,452	$ 72,726	$ 68,568	$ 58,816
Loss from operations	(16,332)	(17,556)	(18,510)	(4,968)	(5,581)	(18,030)	(16,583)	31,043	(57,366)	(9,151)	(45,990)
Net loss	$ (15,954)	$ (16,878)	$ (17,758)	$ (4,199)	$ (4,735)	$ (17,222)	$ (15,852)	$ 31,212	$ (54,789)	$ (6,597)	$ (45,377)
Net loss per share attributable to common stockholders—basic and diluted	$ (0.57)	$ (0.61)	$ (0.64)	$ (0.15)					$ (1.97)	$ (0.33)	$ (18.35)
Net loss per share attributable to common stockholders—basic					$ (0.17)	$ (0.62)	$ (0.73)	$ 1.59
Net loss per share attributable to common stockholders— diluted					$ (0.17)	$ (0.62)	$ (0.73)	$ 1.05