Income Taxes (Details 2) - USD ($)		12 Months Ended
	Dec. 22, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reasons for the difference between income tax expense and statutory federal income tax
Statutory federal income tax		$ 22,056,000	$ 35,476,000	$ 31,194,000
Tax-exempt interest income		(650,000)	(1,197,000)	(1,235,000)
State taxes, net of federal income tax benefit		3,315,000	2,627,000	2,165,000
Deferred tax liability remeasurement		(875,000)	(2,614,000)	0
Other		(1,233,000)	(983,000)	(784,000)
Total provision		$ 22,613,000	$ 33,309,000	$ 31,340,000
Reasons for the difference between income tax expense rate and statutory federal income tax rate
Statutory federal income tax (as a percent)	21.00%	21.00%	35.00%	35.00%
Tax-exempt interest income (as a percent)		(0.60%)	(1.20%)	(1.40%)
State taxes, net of federal income tax benefit (as a percent)		3.20%	2.60%	2.40%
Deferred tax liability remeasurement (as a percent)		(0.80%)	(2.60%)	0.00%
Other (as a percent)		(1.30%)	(0.90%)	(0.80%)
Total income tax expense (benefit) (as a percent)		21.50%	32.90%	35.20%
Deferred tax assets:
Reserve for loan and lease losses		$ 25,386,000	$ 23,791,000
Accruals for employee benefits		2,974,000	2,369,000
Net unrealized losses on securities available-for-sale		3,386,000	1,285,000
Other		127,000	622,000
Total deferred tax assets		31,873,000	28,067,000
Deferred tax liabilities:
Differing depreciable bases in premises and leased equipment		21,184,000	22,641,000
Differing bases in assets related to acquisitions		4,021,000	3,954,000
Tax advantaged partnerships		4,354,000	1,921,000
Mortgage servicing		586,000	745,000
Capitalized loan costs		1,110,000	867,000
Prepaid expenses		273,000	387,000
Other		364,000	222,000
Total deferred tax liabilities		31,892,000	30,737,000
Net deferred tax liability		(19,000)	(2,670,000)
Reconciliation of the beginning and ending amounts of unrecognized tax benefits
Balance, at beginning of the period		1,112,000	762,000	$ 380,000
Additions based on tax positions related to the current year		0	350,000	382,000
Additions for tax positions of prior years		0	0	0
Reductions for tax positions of prior years		0	0	0
Reductions due to lapse in statute of limitations		0	0	0
Settlements		(1,112,000)	0	0
Balance at end of the period		0	1,112,000	762,000
Valuation allowance for deferred tax assets		0	0
Unrecognized tax benefits that would affect the effective tax rate if recognized		0	720,000	500,000
Interest and penalties net of tax recognized		(90,000)	50,000	40,000
Accrued interest and penalties		0	90,000	40,000
Investment securities available-for-sale
Reasons for the difference between income tax expense and statutory federal income tax
Total provision		$ (83,000)	$ 1,629,000	$ 674,000