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
Mar. 31, 2019	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Federal taxes calculated at statutory rate	$ 5,368	$ 5,300
Federal taxes calculated at statutory rate, percent	21.00%	21.00%
Increase (decrease) resulting from:
State taxes, net of federal benefit	$ 1,138	$ 1,143
Benefit of equity based compensation	(392)	(736)
Municipal interest income, net of interest disallowance	(216)	(201)
Bank owned life insurance	(12)	(12)
Other	89	(12)
Total	$ 5,975	$ 5,482
Effective Income Tax Rate Reconciliation, Percent [Abstract]
State taxes, net of federal benefit, percent	4.50%	4.50%
Benefit of equity based compensation, percent	(1.50%)	(3.00%)
Municipal interest income, net of interest disallowance, percent	(0.80%)	(0.80%)
Bank owned life insurance, percent	(0.00%)	(0.00%)
Other, percent	0.20%	0.00%
Income tax expense, as reported	23.40%	21.70%

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

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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 permanent items.

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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 domestic federal statutory tax rate applicable to pretax income (loss).

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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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