Basis of presentation - Schedule of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Basic earnings per common share calculation:
Net income	$ 22,873	$ 18,688	$ 23,618	$ 38,276
Dividends paid on and undistributed earnings allocated to participating securities	0	(100)	0	(205)
Earnings attributable to common shareholders	$ 22,873	$ 18,588	$ 23,618	$ 38,071
Weighted-average basic shares outstanding (in shares)	32,094,274	30,859,596	31,676,004	30,823,341
Basic earnings per common share (in dollars per share)	$ 0.71	$ 0.60	$ 0.75	$ 1.24
Diluted earnings per common share:
Earnings attributable to common shareholders	$ 22,873	$ 18,588	$ 23,618	$ 38,071
Weighted-average basic shares outstanding (in shares)	32,094,274	30,859,596	31,676,004	30,823,341
Weighted-average diluted shares contingently issuable (in shares)	412,143	518,422	433,190	525,625
Weighted-average diluted shares outstanding (in shares)	32,506,417	31,378,018	32,109,194	31,348,966
Diluted earnings per common share (in dollars per share)	$ 0.70	$ 0.59	$ 0.74	$ 1.21
Restricted stock units outstanding considered to be antidilutive (in shares)	352,888		352,888