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, 2019	Dec. 31, 2018	Dec. 31, 2017
Aggregate intrinsic value
Weighted-average fair value of options granted	$ 9.09	$ 7.19	$ 4.52
Share-based compensation expense	$ 6,388	$ 4,884	$ 2,168
Outstanding stock options
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)	2,476,911
Granted (in shares)	882,959
Exercised (in shares)	(414,672)
Cancelled or forfeited (in shares)	(438,136)
Outstanding at the end of the period (in shares)	2,507,062	2,476,911
Vested and expected to vest at the end of the period (in shares)	2,507,062
Exercisable at the end of the period (in shares)	1,284,900
Weighted-average exercise price
Outstanding at the beginning of the period (in dollars per share)	$ 9.65
Granted (in dollars per share)	24.20
Exercised (in dollars per share)	6.80
Cancelled or forfeited (in dollars per share)	14.37
Outstanding at the end of the period (in dollars per share)	14.41	$ 9.65
Vested and expected to vest at the end of the period (in dollars per share)	14.41
Exercisable at the end of the period (in dollars per share)	$ 8.70
Remaining contractual life
Outstanding (in years)	7 years 6 months 29 days	7 years 8 months 23 days
Vested and expected to vest at the end of the period (in years)	7 years 6 months 29 days
Exercisable at the end of the period (in years)	6 years 5 months 16 days
Aggregate intrinsic value
Outstanding at the beginning of the period	$ 22,108
Exercised	6,900	$ 5,300	1,100
Outstanding at the end of the period	24,870	$ 22,108
Vested and expected to vest at the end of the period	24,870
Exercisable at the end of the period	$ 19,239
Weighted-average fair value of options granted	$ 9.09	$ 7.19
Share-based compensation expense	$ 3,700	$ 2,700	1,500
Intrinsic value of stock options exercised	6,900	$ 5,300	$ 1,100
Total unrecognized compensation cost related to unvested stock options	$ 8,700
Period of recognition of unrecognized compensation cost	2 years 9 months 7 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
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
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%