Warrants, stock-based compensation, stock options, restricted stock and restricted stock units - Stock options (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Aggregate intrinsic value
Share-based compensation expense	$ 1,828	$ 1,867	$ 4,713	$ 3,393
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)			818,918
Exercised (in shares)			(322,250)
Cancelled or forfeited (in shares)			(311,245)
Outstanding at the end of the period (in shares)	2,662,334		2,662,334		2,476,911
Vested and expected to vest at the end of the period (in shares)	2,662,334		2,662,334
Exercisable at the end of the period (in shares)	1,234,533		1,234,533
Weighted-average exercise price
Outstanding at the beginning of the period (in dollars per share)			$ 9.65
Granted (in dollars per share)			24.25
Exercised (in dollars per share)			6.74
Cancelled or forfeited (in dollars per share)			13.45
Outstanding at the end of the period (in dollars per share)	$ 14.04		14.04		$ 9.65
Vested and expected to vest at the end of the period (in dollars per share)	14.04		14.04
Exercisable at the end of the period (in dollars per share)	$ 7.91		$ 7.91
Remaining contractual life
Outstanding (in years)			7 years 8 months 19 days		7 years 8 months 23 days
Vested and expected to vest at the end of the period (in years)			7 years 8 months 19 days
Exercisable at the end of the period (in years)			6 years 4 months 24 days
Aggregate intrinsic value
Outstanding at the beginning of the period			$ 22,108
Exercised	$ 1,300	600	5,500	$ 3,400
Outstanding at the end of the period	23,476		23,476		$ 22,108
Vested and expected to vest at the end of the period	23,476		23,476
Exercisable at the end of the period	17,394		$ 17,394
Weighted-average fair value of options granted			$ 8.85	$ 7.32
Share-based compensation expense	1,000	700	$ 2,700	$ 1,800
Intrinsic value of stock options exercised	$ 1,300	$ 600	$ 5,500	$ 3,400
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%