Quarterly Information (Unaudited) - Quarterly Unaudited Statements of Operations (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]
Revenue	$ 12,905	$ 11,289	$ 10,004	$ 7,419	$ 6,675	$ 3,259	$ 3,187	$ 388	$ 41,617	$ 13,509	$ 539
Total operating expenses	24,762	23,327	20,933	26,871	24,715	22,560	19,154	18,318	95,893	84,747	70,547
Loss from operations	(11,857)	(12,038)	(10,929)	(19,452)	(18,040)	(19,301)	(15,967)	(17,930)	(54,276)	(71,238)	(70,008)
Net loss	$ (19,632)	$ (13,962)	$ (12,612)	$ (20,539)	$ (19,826)	$ (19,921)	$ (16,680)	$ (18,669)	$ (66,745)	$ (75,096)	$ (73,673)
Basic and diluted net loss per share (in dollars per share)	$ (0.45)	$ (0.34)	$ (0.32)	$ (0.54)	$ (0.52)	$ (0.53)	$ (0.44)	$ (0.51)	$ (1.65)	$ (2.00)	$ (2.22)