Net Loss Per Share - Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share	15,699	20,127	26,849
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	15,668	20,057	26,608
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	31	70	57
Warrants 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			184