Summary of Quarterly Data (Unaudited) (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 Data [Abstract]
Revenue	$ 14	$ 12	$ 12	$ 6	$ 15	$ 10	$ 91	$ 120	$ 44	$ 236	$ 641
Operating expenses	25,157	32,787	34,977	31,622	29,938	46,467	130,486	32,221	124,544	239,112	6,216
Operating loss	(25,143)	(32,775)	(34,965)	(31,616)	(29,923)	(46,457)	(130,395)	(32,101)	(124,500)	(238,876)	(5,575)
Net loss	$ (24,238)	$ (31,897)	$ (33,997)	$ (30,677)	$ (26,822)	$ (46,312)	$ (130,893)	$ (32,849)	$ (120,809)	$ (236,876)	$ (6,185)
Net loss per share - basic and diluted	$ (0.29)	$ (0.39)	$ (0.41)	$ (0.38)	$ (0.33)	$ (0.60)	$ (1.99)	$ (0.54)	$ (1.47)	$ (3.31)	$ (0.75)
Shares used in calculating net loss per share - basic and diluted	82,235,571	82,154,219	81,959,248	81,574,709	81,247,430	77,837,586	65,789,041	61,137,625	81,979,005	71,519,609	8,246,028