Net Loss Per Share - Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share	18,516	25,059
Equity Awards to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share	18,459	24,933
Unvested Shares of Common Stock Issued Upon Early Exercise of Stock Options and Business Acquisition
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share	57	126