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, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal taxes calculated at statutory rate	$ 22,230	$ 25,720	$ 5,061
Federal taxes calculated at statutory rate, percent	21.00%	35.00%	8.10%
Increase (decrease) resulting from:
State taxes, net of federal benefit	$ 4,666	$ 3,053	$ 3,664
Revaluation of net deferred tax liability as a result of the Tax Cuts and Jobs Act	0	(5,894)	0
Conversion as of September 16, 2016 to C Corporation	0	0	13,181
Benefit of equity based compensation	(870)	(310)	(786)
Municipal interest income, net of interest disallowance	(837)	(1,402)	(633)
Bank owned life insurance	(51)	(85)	(24)
Stock offering costs	141	0	0
Other	339	5	1,270
Total	$ 25,618	$ 21,087	$ 21,733
Effective Income Tax Rate Reconciliation, Percent [Abstract]
State taxes, net of federal benefit, percent	4.40%	4.20%	5.90%
Revaluation of net deferred tax liability as a result of the Tax Cuts and Jobs Act, percent	(0.00%)	(8.00%)	(0.00%)
Conversion as of September 16, 2016 to C Corporation, percent	0.00%	0.00%	21.10%
Benefit of equity based compensation, percent	(0.80%)	(0.40%)	(1.30%)
Municipal interest income, net of interest disallowance, percent	(0.80%)	(1.90%)	(1.00%)
Bank owned life insurance, percent	(0.00%)	(0.20%)	(0.00%)
Stock offering costs, percent	0.10%	0.00%	0.00%
Other, percent	0.30%	0.00%	2.10%
Income tax expense, as reported	24.20%	28.70%	34.90%