Reconciliation of Numerator and Denominator Used in Computation of Basic and Diluted Net Income Loss per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Numerator for Basic and Diluted:
Net loss used for basic calculation	$ (13,501)	$ (14,376)	$ (18,166)	$ (16,863)	$ (14,756)	$ (15,680)	$ (15,972)	$ (9,460)	$ (62,906)	$ (55,868)	$ (38,755)
Effect of revaluation of warrant liability										(3,566)	(7,708)
Adjusted net loss used for dilution calculation									$ (62,906)	$ (59,434)	$ (46,463)
Denominator:
Basic weighted average number of shares outstanding									101,826	96,068	74,767
Effect of dilutive potential shares										837	1,767
Diluted weighted average number of shares outstanding									101,826	96,905	76,534
Net loss per share:
Basic	$ (0.13)	$ (0.14)	$ (0.18)	$ (0.17)	$ (0.15)	$ (0.16)	$ (0.17)	$ (0.10)	$ (0.62)	$ (0.58)	$ (0.52)
Diluted	$ (0.13)	$ (0.14)	$ (0.18)	$ (0.17)	$ (0.15)	$ (0.17)	$ (0.17)	$ (0.17)	$ (0.62)	$ (0.61)	$ (0.61)