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	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]
Total operating expenses	$ 56,379	$ 38,064	$ 35,006	$ 30,714	$ 28,612	$ 24,082	$ 25,059	$ 16,897	$ 160,163	$ 94,650	$ 33,810
Loss from operations	(56,379)	(38,064)	(35,006)	(30,714)	(28,612)	(24,082)	(25,059)	(16,897)	(160,163)	(94,650)	(33,810)
Net loss	$ (55,901)	$ (37,796)	$ (34,747)	$ (30,543)	$ (28,562)	$ (24,035)	$ (25,027)	$ (16,871)	$ (158,987)	$ (94,495)	$ (33,811)
Net loss per share attributable to common stockholders—basic and diluted	$ (1.50)	$ (1.15)	$ (1.08)	$ (0.97)	$ (0.99)	$ (0.84)	$ (0.90)	$ (0.66)	$ (4.75)	$ (3.40)	$ (1.67)