Basis of presentation - Schedule of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic earnings per common share calculation:
Net income	$ 83,814	$ 80,236	$ 52,398
Dividends paid on and undistributed earnings allocated to participating securities	(447)	(428)	0
Earnings attributable to common shareholders	$ 83,367	$ 79,808	$ 52,398
Weighted-average basic shares outstanding (in shares)	30,870,474	30,675,755	27,627,228
Basic earnings per common share (in dollars per share)	$ 2.70	$ 2.60	$ 1.90
Diluted earnings per common share:
Earnings attributable to common shareholders	$ 83,367	$ 79,808	$ 52,398
Weighted-average basic shares outstanding (in shares)	30,870,474	30,675,755	27,627,228
Weighted-average diluted shares contingently issuable (in shares)	532,423	639,226	580,374
Weighted-average diluted shares outstanding (in shares)	31,402,897	31,314,981	28,207,602
Diluted earnings per common share (in dollars per share)	$ 2.65	$ 2.55	$ 1.86