Net Loss Per Share - Number of Outstanding Stock Options, Warrants to Purchase Common Preferred Stock and Convertible Redeemable Preferred Stock Excluded from Computation of Diluted Net Income (Loss) Per Share (Detail) (Excluded From Earnings Per Share Calculation)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Excluded From Earnings Per Share Calculation
   
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]    
Stock options outstanding 2,898,461 5,131,673
Restricted stock units and restricted stock awards 145,676 217,010
Common equivalent shares from stock warrants 2,219,671 2,378,641
Shares purchased under ESPP 0 25,105
Antidilutive securities excluded from computation of earnings per share, amount, total 5,263,808 7,752,429