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 Disclosure [Abstract]
Collaboration revenue	$ 77,016	$ 211,928	$ 318	$ 328	$ 115	$ 563	$ 1,088	$ 1,358	$ 289,590	[1]	$ 3,124	[1]	$ 40,997	[1]
Total operating expenses	66,033	72,765	55,301	48,751	45,343	49,995	38,374	28,355	242,850		162,067		105,645
(Loss) income from operations	10,983	139,163	(54,983)	(48,423)	(45,228)	(49,432)	(37,286)	(26,997)	46,740		(158,943)		(64,648)
Net income (loss)	$ 30,542	$ 138,423	$ (53,699)	$ (48,408)	$ (47,590)	$ (50,711)	$ (38,380)	$ (28,300)	$ 66,858		$ (164,981)		$ (68,357)
Net (loss) income per share attributable to common shareholders:
Basic	$ 0.53	$ 2.52	$ (1.01)	$ (0.93)	$ (0.92)	$ (1.07)	$ (0.82)	$ (0.62)	$ 1.23		$ (3.44)		$ (1.71)
Diluted	$ 0.51	$ 2.40	$ (1.01)	$ (0.93)	$ (0.92)	$ (1.07)	$ (0.82)	$ (0.62)	$ 1.17		$ (3.44)		$ (1.71)
Weighted-average common shares outstanding used in net (loss) income per share attributable to common shareholders:
Basic	57,395,839	54,829,057	53,188,041	52,093,208	51,688,383	47,391,988	46,842,316	45,877,428	54,392,304		47,964,368		40,057,365
Diluted	60,233,927	57,598,901	53,188,041	52,093,208	51,688,383	47,391,988	46,842,316	45,877,428	56,932,798		47,964,368		40,057,365

[1] Including the following amounts of revenue from a related party, see Note 7