Basis of presentation - Schedule of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Basic earnings per common share calculation:
Net income	$ 23,966	$ 21,377	$ 62,242	$ 63,196
Dividends paid on and undistributed earnings allocated to participating securities	(128)	(114)	(333)	(337)
Earnings attributable to common shareholders	$ 23,838	$ 21,263	$ 61,909	$ 62,859
Weighted-average basic shares outstanding (in shares)	30,899,583	30,692,668	30,849,035	30,661,852
Basic earnings per common share (in dollars per share)	$ 0.77	$ 0.69	$ 2.01	$ 2.05
Diluted earnings per common share:
Earnings attributable to common shareholders	$ 23,838	$ 21,263	$ 61,909	$ 62,859
Weighted-average basic shares outstanding (in shares)	30,899,583	30,692,668	30,849,035	30,661,852
Weighted-average diluted shares contingently issuable (in shares)	525,990	646,960	529,751	636,802
Weighted-average diluted shares outstanding (in shares)	31,425,573	31,339,628	31,378,786	31,298,654
Diluted earnings per common share (in dollars per share)	$ 0.76	$ 0.68	$ 1.97	$ 2.01