Income taxes - Schedule of Reconciliation of Income Taxes Computed at the United States Federal Statutory Tax Rates to the Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal taxes calculated at statutory rate	$ 23,003	$ 22,230	$ 25,720
Federal taxes calculated at statutory rate, percent	21.00%	21.00%	35.00%
Increase (decrease) resulting from:
State taxes, net of federal benefit	$ 4,792	$ 4,666	$ 3,053
Revaluation of net deferred tax liability as a result of the Tax Cuts and Jobs Act	0	0	(5,894)
Benefit of equity based compensation	(1,353)	(870)	(310)
Municipal interest income, net of interest disallowance	(908)	(837)	(1,402)
Bank owned life insurance	(51)	(51)	(85)
Stock offering costs	66	141	0
Other	176	339	5
Total	$ 25,725	$ 25,618	$ 21,087
Effective Income Tax Rate Reconciliation, Percent [Abstract]
State taxes, net of federal benefit, percent	4.40%	4.40%	4.20%
Revaluation of net deferred tax liability as a result of the Tax Cuts and Jobs Act, percent	0.00%	0.00%	(8.00%)
Benefit of equity based compensation, percent	(1.20%)	(0.80%)	(0.40%)
Municipal interest income, net of interest disallowance, percent	(0.80%)	(0.80%)	(1.90%)
Bank owned life insurance, percent	(0.10%)	(0.00%)	(0.20%)
Stock offering costs, percent	0.10%	0.10%	0.00%
Other, percent	0.10%	0.30%	0.00%
Income tax expense, as reported	23.50%	24.20%	28.70%