Stock-based compensation - Restricted Stock - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Restricted stock disclosures
Stock-based compensation.	$ 3,601	$ 4,040
Unvested restricted common stock units
Shares
Balance, beginning (in shares)	638,471
Granted (in shares)	1,203,684
Vested (in shares)	(184,217)
Forfeited (in shares)	(50,607)
Balance, ending (in shares)	1,607,331
Weighted Average Grant Date Fair Value Per Share
Balance, beginning (in dollars per share)	$ 12.74
Granted (in dollars per share)	6.28
Vested (in dollars per share)	11.55
Forfeited (in dollars per share)	9.40
Balance, ending (in dollars per share)	$ 8.15
Restricted stock disclosures
Vesting period	3 years	3 years
Stock-based compensation.	$ 975	$ 741
Unrecognized stock-based compensation expense	$ 11,800
Remaining weighted-average remaining vesting period	2 years 2 months 12 days
Vesting period, tranche one | Unvested restricted common stock units
Shares
Granted (in shares)	503,684
Restricted stock disclosures
Vesting period	3 years
Vesting period, tranche two | Unvested restricted common stock units
Shares
Granted (in shares)	700,000
Restricted stock disclosures
Vesting period	2 years