Net Loss Per Share - Schedule of Potential Common Shares Issuable Upon Conversion of Warrants (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Potential common shares
Total (in shares)	2,833,619	2,225,648	12,137,409
Stock options to purchase common stock
Potential common shares
Total (in shares)	2,657,187	2,128,346	1,569,379
Stock warrants to purchase common stock
Potential common shares
Total (in shares)	176,432	97,302	650,618
Redeemable convertible preferred stock
Potential common shares
Total (in shares)	0	0	9,890,580
Redeemable convertible preferred stock warrants
Potential common shares
Total (in shares)	0	0	26,832