Income Taxes (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current:
Federal	$ 25,479,000	$ 26,092,000	$ 20,999,000
State	3,005,000	3,365,000	1,034,000
Total current	28,484,000	29,457,000	22,033,000
Deferred:
Federal	2,530,000	1,577,000	4,022,000
State	326,000	43,000	319,000
Total deferred	2,856,000	1,620,000	4,341,000
Total income tax expense (benefit)	31,340,000	31,077,000	26,374,000
Reasons for the difference between income tax expense and statutory federal income tax
Statutory federal income tax	31,194,000	30,997,000	29,555,000
Tax-exempt interest income	(1,235,000)	(1,152,000)	(1,236,000)
State taxes, net of federal income tax benefit	2,165,000	2,215,000	2,300,000
Reduction in uncertain tax positions	0	0	(3,300,000)
Other	(784,000)	(983,000)	(945,000)
Total income tax expense (benefit)	$ 31,340,000	$ 31,077,000	$ 26,374,000
Reasons for the difference between income tax expense rate and statutory federal income tax rate
Statutory federal income tax (as a percent)	35.00%	35.00%	35.00%
Tax-exempt interest income (as a percent)	(1.40%)	(1.30%)	(1.50%)
State taxes, net of federal income tax benefit (as a percent)	2.40%	2.50%	2.70%
Reduction in uncertain tax positions (as a percent)	0.00%	0.00%	(3.90%)
Other (as a percent)	(0.80%)	(1.10%)	(1.10%)
Total income tax expense (benefit) (as a percent)	35.20%	35.10%	31.20%
Deferred tax assets:
Reserve for loan and lease losses	$ 34,663,000	$ 34,410,000
Accruals for employee benefits	3,948,000	3,816,000
Tax advantaged partnerships	1,411,000	307,000
Other	477,000	598,000
Total deferred tax assets	40,499,000	39,131,000
Deferred tax liabilities:
Differing depreciable bases in premises and leased equipment	31,449,000	27,274,000
Net unrealized gains on securities available-for-sale	807,000	3,940,000
Differing bases in assets related to acquisitions	6,170,000	5,738,000
Mortgage servicing	1,540,000	1,630,000
Capitalized loan costs	1,463,000	1,454,000
Prepaid expenses	646,000	1,055,000
Other	419,000	312,000
Total deferred tax liabilities	42,494,000	41,403,000
Net deferred tax liability	(1,995,000)	(2,272,000)
Reconciliation of the beginning and ending amounts of unrecognized tax benefits
Balance, at beginning of the period	380,000	0	$ 4,611,000
Additions based on tax positions related to the current year	382,000	380,000	66,000
Additions for tax positions of prior years	0	0	592,000
Reductions for tax positions of prior years	0	0	(553,000)
Reductions due to lapse in statute of limitations	0	0	(1,650,000)
Settlements	0	0	(3,066,000)
Balance at end of the period	762,000	380,000	0
Valuation allowance for deferred tax assets	0	0
Unrecognized tax benefits that would affect the effective tax rate if recognized	500,000	250,000	0
Interest and penalties net of tax recognized	40,000	0	(690,000)
Accrued interest and penalties	40,000	0	0
Increase in earnings from settlement of unrecognized tax benefits			2,120,000
Investment securities available-for-sale
Deferred:
Total income tax expense (benefit)	674,000	2,000	361,000
Reasons for the difference between income tax expense and statutory federal income tax
Total income tax expense (benefit)	$ 674,000	$ 2,000	361,000
State and local jurisdiction
Reconciliation of the beginning and ending amounts of unrecognized tax benefits
Settlements			$ (2,930,000)