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
	Mar. 31, 2019	Mar. 31, 2018
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	10,663,223	9,630,761
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	1,088,699	1,527,584
Warrants [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from computation of net loss per share		301,504