Stock-based compensation - Restricted Stock - (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restricted stock disclosures
Stock-based compensation.	$ 14,934	$ 15,640	$ 15,710
Unvested restricted common stock units
Shares
Balance, beginning (in shares)	455,404
Granted (in shares)	494,604
Vested (in shares)	(150,759)
Forfeited (in shares)	(160,778)
Balance, ending (in shares)	638,471	455,404
Weighted Average Grant Date Fair Value Per Share
Balance, beginning (in dollars per share)	$ 12.16
Granted (in dollars per share)	12.67
Vested (in dollars per share)	12.12
Forfeited (in dollars per share)	11.47
Balance, ending (in dollars per share)	$ 12.74	$ 12.16
Restricted stock disclosures
Vesting period	3 years
Stock-based compensation.	$ 3,110	$ 1,935	482
Unrecognized stock-based compensation expense	$ 5,600
Remaining weighted-average remaining vesting period	1 year 9 months 18 days
Share-based Payment Arrangement, Employee
Restricted stock disclosures
Stock-based compensation.	$ 11,200	13,300	14,600
Share-based Payment Arrangement, Employee | Unvested restricted common stock units
Restricted stock disclosures
Stock-based compensation.	$ 2,800	$ 1,900	$ 100