Warrants to Purchase Convertible Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Jul. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2015
Warrants to purchase convertible preferred stock
Other expenses		$ 2.2	$ 0.8
IPO
Warrants to purchase convertible preferred stock
Warrants to purchase aggregate	135,567
Issuance of warrants exercise	131,273
Shares withheld to pay exercise price	4,294
Series A redeemable convertible preferred stock
Warrants to purchase convertible preferred stock
Maximum number of preferred shares can purchase from outstanding warrants				133,333
Warrants exercisable price				$ 0.60
Warrants contractual term (in years)				10 years
Series B convertible preferred stock
Warrants to purchase convertible preferred stock
Maximum number of preferred shares can purchase from outstanding warrants			2,234
Warrants exercisable price			$ 8.39
Warrants contractual term (in years)			10 years