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
Warrants contractual term (in years)			10 years
Other expenses		$ 2.2
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