Convertible Notes, Convertible Preferred Stock and Common and Preferred Stock Warrants - Preferred stock Warrants (Additional Information) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Warrant or Right [Line Items]
Gain on change in fair value of warrant	$ 8,900	$ 3,100	$ 800
Reclassification of redeemable convertible preferred stock warrant liability to additional paid-in capital	$ 882	$ 0	$ 0
Preferred stock warrants
Class of Warrant or Right [Line Items]
Warrants outstanding (in shares)	0	860,788