Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Detail) - 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 [Line Items]
Collaboration revenue - related party									$ 27,188	$ 26,917	$ 32,100
Grant revenue	$ 311	$ 85	$ 260	$ 446	$ 433	$ 371	$ 341	$ 205	1,102	1,350
Collaboration revenue	2,032	2,106	1,817	260					6,215
Total operating expenses	26,814	24,214	23,479	31,874	32,300	31,266	32,748	32,237	106,381	128,551	123,495
Loss from operations	(19,192)	(17,183)	(10,948)	(24,553)	(21,671)	(22,211)	(28,136)	(28,266)	(71,876)	(100,284)	(91,395)
Net loss	$ (18,778)	$ (16,409)	$ (10,759)	$ (24,333)	$ (21,287)	$ (21,949)	$ (27,787)	$ (27,919)	$ (70,279)	$ (98,942)	$ (89,380)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.27)	$ (0.23)	$ (0.24)	$ (0.59)	$ (0.52)	$ (0.54)	$ (0.68)	$ (0.69)	$ (1.24)	$ (2.43)	$ (2.21)
Collaboration Revenue - Related Party [Member]
Quarterly Financial Information [Line Items]
Collaboration revenue - related party	$ 5,279	$ 4,840	$ 10,454	$ 6,615	$ 10,196	$ 8,684	$ 4,271	$ 3,766	$ 27,188	$ 26,917