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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Basic earnings per common share calculation:
Net income	$ 18,688	$ 22,065	$ 38,276	$ 41,819
Dividends paid on and undistributed earnings allocated to participating securities	(100)	(117)	(205)	(223)
Earnings attributable to common shareholders	$ 18,588	$ 21,948	$ 38,071	$ 41,596
Weighted-average basic shares outstanding (in shares)	30,859,596	30,678,732	30,823,341	30,646,189
Basic earnings per common share (in dollars per share)	$ 0.60	$ 0.72	$ 1.24	$ 1.36
Diluted earnings per common share:
Earnings attributable to common shareholders	$ 18,588	$ 21,948	$ 38,071	$ 41,596
Weighted-average basic shares outstanding (in shares)	30,859,596	30,678,732	30,823,341	30,646,189
Weighted-average diluted shares contingently issuable (in shares)	518,422	615,312	525,625	629,657
Weighted-average diluted shares outstanding (in shares)	31,378,018	31,294,044	31,348,966	31,275,846
Diluted earnings per common share (in dollars per share)	$ 0.59	$ 0.70	$ 1.21	$ 1.33