Convertible Notes, Convertible Preferred Stock and Common and Preferred Stock Warrants - Preferred stock Warrants (Additional Information) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Class of Warrant or Right [Line Items]
Gain on change in fair value of warrant	$ 1,487	$ 8,943
Reclassification of redeemable convertible preferred stock warrant liability to additional paid-in capital	$ 882	$ 882	$ 0
Preferred stock warrants
Class of Warrant or Right [Line Items]
Warrants outstanding (in shares)	0	0		860,788
Warrants and Rights Outstanding	$ 0	$ 0		$ 9,825