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, 2016	[1]	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue	$ 2,344		$ 1,549	$ 795	$ 476	$ 247				$ 5,164	[2]	$ 247	[2]
Total operating expenses	27,654		16,159	17,353	12,128	12,413	$ 6,202	$ 3,625	$ 3,736	73,294		25,976		$ 6,627
Loss from operations	(25,310)		(14,610)	(16,558)	(11,652)	(12,166)	(6,202)	(3,625)	(3,736)	(68,130)		(25,729)		(6,627)
Net loss	17,098		(14,694)	(17,164)	(8,442)	(12,286)	(6,354)	(3,666)	(3,522)	(23,202)		(25,828)		(6,800)
Net (loss) income attributable to common shareholders	$ 17,099		$ (14,680)	$ (17,157)	$ (8,439)	$ (12,270)	$ (6,353)	$ (3,643)	$ (3,237)	$ (23,177)		$ (25,503)		$ (7,009)
Net (loss) income per share attributable to common shareholders:
Basic	$ 0.43		$ (2.77)	$ (3.15)	$ (1.53)
Diluted	$ 0.40		$ (2.77)	$ (3.15)	$ (1.53)
Net loss per share attributable to common shareholders—basic and diluted						$ (2.22)	$ (1.15)	$ (0.80)	$ (0.91)	$ (1.89)		$ (5.06)		$ 1.97
Weighted-average common shares outstanding used in net (loss) income per share attributable to common shareholders:
Basic	32,987,335		5,292,348	5,448,855	5,528,079
Diluted	34,989,218		5,292,348	5,448,855	5,528,079
Weighted-average common shares outstanding used in net loss per share attributable to common shareholders—basic and diluted						5,528,079	5,528,079	4,538,595	3,560,000	12,257,483		5,037,404		3,559,985

[1] During the fourth quarter the Company recorded an immaterial correction of an error of $1.2 million for rent expense related to the three months ended September 30, 2016. The Company determined that these errors are not material to the respective interim financial statements.

[2] Including the following amounts of revenue from a related party, see Note 16: