Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Shares that were Excluded from Computation of Diluted Net Loss per Share of Common Stock (Detail) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Stock-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	9,925,584	9,038,397	9,815,543	10,434,764
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	1,500,618	2,792,247	1,503,118	2,972,278
Warrants [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from computation of net loss per share	270,027	801,673	286,010	736,567