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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Data [Abstract]
Collaboration revenue - related party	[1]	$ 3,056	$ 23,015	$ 3,014	$ 3,015	$ 3,037	$ 13,015	$ 3,004	$ 2,710	$ 32,100	$ 21,766
Total operating expenses		32,831	30,329	31,430	28,905	28,725	32,110	31,144	22,626	123,495	114,605	$ 54,856
Loss from operations		(29,775)	(7,314)	(28,416)	(25,890)	(25,688)	(19,095)	(28,140)	(19,916)	(91,395)	(92,839)	(54,856)
Net loss		$ (28,953)	$ (6,935)	$ (28,018)	$ (25,474)	$ (25,274)	$ (18,688)	$ (27,913)	$ (19,704)	$ (89,380)	$ (91,579)	$ (54,780)
Net loss per share attributable to common stockholders, basic and diluted		$ (0.72)	$ (0.17)	$ (0.69)	$ (0.63)	$ (0.63)	$ (0.46)	$ (0.70)	$ (0.50)	$ (2.21)	$ (2.30)	$ (2.33)

[1] In January 2016, the Company entered into a Collaboration and License Agreement with NHS for the development and commercialization of certain of its product candidates. In exchange for the license, NHS paid the Company an upfront cash payment of $120,000, which the Company received in February 2016 and is being recognized as revenue over the ten year performance period. During the quarter ended September 30, 2016, the Company received $10,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2016. During the quarter ended September 30, 2017, the Company received $20,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2017.