Basis of presentation - Schedule of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic earnings per common share calculation:
Net income	$ 80,236	$ 52,398	$ 40,591
Dividends paid on and undistributed earnings allocated to participating securities	(428)	0	0
Earnings attributable to common shareholders	$ 79,808	$ 52,398	$ 40,591
Weighted-average basic shares outstanding (in shares)	30,675,755	27,627,228	19,165,182
Basic earnings per share (in dollars per share)	$ 2.60	$ 1.90	$ 2.12
Diluted earnings per common share:
Earnings attributable to common shareholders	$ 79,808	$ 52,398	$ 40,591
Weighted-average basic shares outstanding (in shares)	30,675,755	27,627,228	19,165,182
Weighted-average diluted shares contingently issuable (in shares)	639,226	580,374	146,992
Weighted-average diluted shares outstanding (in shares)	31,314,981	28,207,602	19,312,174
Diluted earnings per share (in dollars per share)	$ 2.55	$ 1.86	$ 2.10
Pro Forma, Earnings Attributable To Common Shareholders	$ 79,808	$ 52,398	$ 39,422
Pro forma earnings per share:
Weighted-average basic shares outstanding, pro-forma (in shares)	30,675,755	27,627,228.00	19,165,182.00
Pro forma basic earnings per share (in dollars per share)	$ 2.60	$ 1.90	$ 2.06
Pro forma diluted earnings per share:
Pro forma net income	$ 80,236	$ 52,398	$ 39,422
Weighted-average diluted shares outstanding, pro forma (in shares)	31,314,981	28,207,602	19,312,174
Pro forma diluted earnings per share (in dollars per share)	$ 2.55	$ 1.86	$ 2.04