Warrants, stock-based compensation, stock options, restricted stock and restricted stock units - Stock options (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Aggregate intrinsic value
Share-based compensation expense	$ 2,109	$ 1,284
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)	2,507,062
Granted (in shares)	428,271
Exercised (in shares)	(63,074)
Cancelled or forfeited (in shares)	(31,734)
Outstanding at the end of the period (in shares)	2,840,525		2,507,062
Vested and expected to vest at the end of the period (in shares)	2,840,525
Exercisable at the end of the period (in shares)	1,411,836
Weighted-average exercise price
Outstanding at the beginning of the period (in dollars per share)	$ 14.41
Granted (in dollars per share)	27.16
Exercised (in dollars per share)	7.86
Cancelled or forfeited (in dollars per share)	24.29
Outstanding at the end of the period (in dollars per share)	16.37		$ 14.41
Vested and expected to vest at the end of the period (in dollars per share)	16.37
Exercisable at the end of the period (in dollars per share)	$ 9.93
Remaining contractual life
Outstanding (in years)	7 years 9 months		7 years 6 months 29 days
Vested and expected to vest at the end of the period (in years)	7 years 9 months
Exercisable at the end of the period (in years)	6 years 6 months 15 days
Aggregate intrinsic value
Outstanding at the beginning of the period	$ 24,870
Exercised	1,100	$ 1,200
Outstanding at the end of the period	14,641		$ 24,870
Vested and expected to vest at the end of the period	14,641
Exercisable at the end of the period	$ 12,703
Weighted-average fair value of options granted	$ 11.82	$ 7.90
Share-based compensation expense	$ 1,100	$ 700
Intrinsic value of stock options exercised	$ 1,100	$ 1,200
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%