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	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Federal taxes calculated at statutory rate	$ 6,653	$ 5,897	$ 17,272	$ 17,467
Federal taxes calculated at statutory rate, percent	21.00%	21.00%	21.00%	21.00%
Increase (decrease) resulting from:
State taxes, net of federal benefit	$ 1,512	$ 1,187	$ 3,855	$ 3,873
Benefit of equity based compensation	(275)	(115)	(668)	(866)
Municipal interest income, net of interest disallowance	(211)	(213)	(650)	(621)
Bank owned life insurance	(11)	(13)	(38)	(38)
Stock offering costs			0	141
Other	50	(41)	236	22
Total	$ 7,718	$ 6,702	$ 20,007	$ 19,978
Effective Income Tax Rate Reconciliation, Percent [Abstract]
State taxes, net of federal benefit, percent	4.70%	4.20%	4.70%	4.60%
Benefit of equity based compensation, percent	(0.90%)	(0.40%)	(0.80%)	(1.00%)
Municipal interest income, net of interest disallowance, percent	(0.70%)	(0.80%)	(0.80%)	(0.80%)
Bank owned life insurance, percent	0.10%	(0.00%)	(0.10%)	(0.10%)
Stock offering costs, percent			0.00%	0.20%
Other, percent	0.20%	(0.10%)	0.30%	0.10%
Income tax expense, as reported	24.40%	23.90%	24.30%	24.00%

Effective income tax rate reconciliation, municipal interest income net of interest disallowance, percent.

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Effective income tax rate reconciliation nondeductible expense stock offering costs.

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Income tax reconciliation bank owned life insurance.

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Income tax reconciliation change in state taxes net of federal benefit.

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Income tax reconciliation nondeductible income share based compensation cost.

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to life insurance expense.

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