Summary of Significant Accounting Policies - Schedule of Potential Weighted Common Shares Outstanding that were Excluded from Computation of Basic and Diluted Net Loss per Share of Common Stock (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stock options [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from computation of net loss per share	260,852	13,881,355	25,007,776
Convertible senior notes [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from computation of net loss per share	2,344,998	411,560
Warrant issued in May 2013 [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from computation of net loss per share	921,985
Employee stock purchase plan [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from computation of net loss per share	23,859	23,296	59,763
Restricted stock units [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from computation of net loss per share	88,213	224
DOE warrant [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from computation of net loss per share		1,061,439	2,342,353