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) - shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Employee share based awards [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from computation of net loss per share	15,592,736	14,729,749	13,904,875
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	2,431,265	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	1,049,791	921,985
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