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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, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Numerator for Basic and Diluted:                      
Net loss used for basic calculation $ (11,486) $ (13,418) $ (17,083) $ (18,598) $ (13,501) $ (14,376) $ (18,166) $ (16,863) $ (60,585) $ (62,906) $ (55,868)
Effect of revaluation of warrant liability                     (3,566)
Adjusted net loss used for dilution calculation                 $ (60,585) $ (62,906) $ (59,434)
Denominator:                      
Basic weighted average number of shares outstanding                 108,221 101,826 96,068
Effect of dilutive potential shares                     837
Diluted weighted average number of shares outstanding                 108,221 101,826 96,905
Net loss per share:                      
Basic $ (0.10) $ (0.12) $ (0.16) $ (0.18) $ (0.13) $ (0.14) $ (0.18) $ (0.17) $ (0.56) $ (0.62) $ (0.58)
Diluted $ (0.10) $ (0.12) $ (0.16) $ (0.18) $ (0.13) $ (0.14) $ (0.18) $ (0.17) $ (0.56) $ (0.62) $ (0.61)