Common Stock, warrants, stock-based compensation, stock options, restricted stock and restricted stock units - Stock options (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Aggregate intrinsic value
Weighted-average fair value of options granted		$ 7.19	$ 4.52	$ 2.41
Share-based compensation expense		$ 4,884	$ 2,168	$ 928
Outstanding stock options
Common Stock, warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Options expiration period (in years)		10 years
Number outstanding
Outstanding at the beginning of the period (in shares)		2,249,843	1,119,671
Granted (in shares)		729,224	1,281,135
Exercised (in shares)		(407,901)	(90,265)
Cancelled or forfeited (in shares)		(94,255)	(60,698)
Outstanding at the end of the period (in shares)	2,476,911	2,476,911	2,249,843	1,119,671
Vested and expected to vest at the end of the period (in shares)	2,476,911	2,476,911
Exercisable at the end of the period (in shares)	1,143,480	1,143,480
Weighted-average exercise price
Outstanding at the beginning of the period (in dollars per share)		$ 6.05	$ 2.99
Granted (in dollars per share)		18.36	8.46
Exercised (in dollars per share)		4.57	2.25
Cancelled or forfeited (in dollars per share)		13.16	6.16
Outstanding at the end of the period (in dollars per share)	$ 9.65	9.65	$ 6.05	$ 2.99
Vested and expected to vest at the end of the period (in dollars per share)	9.65	9.65
Exercisable at the end of the period (in dollars per share)	$ 5.68	$ 5.68
Remaining contractual life
Outstanding (in years)		7 years 8 months 24 days	7 years 9 months 18 days	6 years 9 months 18 days
Vested and expected to vest at the end of the period (in years)		7 years 8 months 24 days
Exercisable at the end of the period (in years)	6 years 6 months 5 days
Aggregate intrinsic value
Outstanding	$ 22,108	$ 22,108	$ 34,695	$ 5,796
Vested and expected to vest at the end of the period	22,108	22,108
Exercisable at the end of the period	14,440	$ 14,440
Weighted-average fair value of options granted		$ 7.19	$ 4.52
Share-based compensation expense		$ 2,700	$ 1,500	200
Intrinsic value of stock options exercised		5,300	$ 1,100	$ 400
Total unrecognized compensation cost related to unvested stock options	$ 6,800	$ 6,800
Period of recognition of unrecognized compensation cost		2 years 2 months 23 days
Outstanding stock options | Subject to a four year vesting schedule with 25% vesting on the first anniversary and the remaining vesting ratably on a monthly basis over the remaining three years
Common Stock, warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Vesting period (in years)		4 years
Vesting percentage 1 (as a percent)		25.00%
Outstanding stock options | Subject to vesting with 50% vesting on December 31, 2018 and December 31, 2019
Common Stock, warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Vesting period (in years)		3 years
Vesting percentage 1 (as a percent)		75.00%