Net Loss and the Number of Shares Used to Compute Basic and Diluted Loss per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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	Jun. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net loss	$ (22,465)	$ (10,825)	$ (13,634)	$ (20,941)	$ (27,453)	$ (15,772)	$ (13,354)	$ (29,771)		$ (67,865)	$ (86,350)	$ (115,204)
Preferred stock purchase option												(868)
Preferred stock beneficial conversion features									$ (31,300)			(32,987)
Net loss applicable to common shareholders—basic and diluted										$ (67,865)	$ (86,350)	$ (149,059)
Weighted average shares outstanding—basic and diluted										270,652	211,874	180,654
Net loss per share—basic and diluted										$ (0.25)	$ (0.41)	$ (0.83)