Employee benefit plans - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Shares (in thousands)
Outstanding at beginning of period (shares)	3,963
Granted (shares)	1,025
Exercised (shares)	(357)
Forfeited/Cancelled (shares)	(1,200)
Outstanding at end of period (shares)	3,431	3,963
Weighted-average exercise price
Outstanding at beginning of period (in dollars per share)	$ 10.16
Granted (usd per share)	4.01
Exercised (usd per share)	5.50
Outstanding at end of period (in dollars per share)	$ 8.79	$ 10.16
Weighted Average Remaining Contractual Term (in years)	6 years 6 months	6 years 4 months 6 days
Aggregate intrinsic value (in thousands)	$ 6,259,000	$ 374,000
Vested and Expected to Vest (shares)	3,431
Vested and Expected to Vest - Weighted Average Exercise Price (in dollars per share)	$ 8.79
Vested and Expected to Vest- Weighted Average Remaining Contractual Term	6 years 6 months
Vested and Expected to Vest - Aggregate Intrinsic Value	$ 6,259,000
Exercisable (shares)	2,195
Exercisable - Weighted average exercise price (in dollars per share)	$ 11.06
Exercisable - Weighted Average Remaining Contractual Term	5 years 2 months 4 days
Exercisable - Aggregate intrinsic value	$ 1,893,000
Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price	$ 10.20
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 2.03	$ 3.70	$ 2.95
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value	$ 1,700,000	$ 3,500,000	$ 6,100,000