Net Loss Per Share - Schedule of Potentially Dilutive Securities not Included in Calculation of Diluted Net Loss Per Share (Details) - shares shares in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities (in common stock equivalent shares) excluded from calculation of diluted net loss per share	3,090	17,127
Convertible Preferred Stock Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities (in common stock equivalent shares) excluded from calculation of diluted net loss per share		14,713
Common Stock Option
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities (in common stock equivalent shares) excluded from calculation of diluted net loss per share	3,040	2,275
Unvested Common Stock Subject to Repurchase
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities (in common stock equivalent shares) excluded from calculation of diluted net loss per share	50	139