Stock based compensation - Restricted Stock - (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Restricted stock disclosures
Stock-based compensation	$ 3,859,000	$ 5,254,000	$ 7,900,000	$ 8,771,000
Restricted stock units
Shares
Balance, beginning (in shares)			455,404
Granted (in shares)			448,929
Vested (in shares)			(130,003)
Forfeited (in shares)			(64,600)
Balance, ending (in shares)	709,730		709,730
Weighted Average Grant Date Fair Value Per Share
Balance, beginning (in dollars per share)			$ 12.16
Granted (in dollars per share)			12.83
Vested (in dollars per share)			11.02
Forfeited (in dollars per share)			10.66
Balance, ending (in dollars per share)	$ 12.93		$ 12.93
Restricted stock disclosures
Vesting period	3 years		3 years
Stock-based compensation	$ 864,000	$ 668,000	$ 1,605,000	$ 1,351,000
Unrecognized stock-based compensation expense	$ 7,700,000		$ 7,700,000
Remaining weighted-average remaining vesting period			2 years 3 months 18 days
Share-based Payment Arrangement, Employee | Restricted stock units
Weighted Average Grant Date Fair Value Per Share
Granted (in dollars per share)	$ 8.05	$ 20.74	$ 12.83	$ 10.91
Restricted stock disclosures
Stock-based compensation	$ 900	$ 700,000	$ 1,600,000	$ 1,400,000