Income Taxes (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current:
Federal	$ 26,092,000	$ 20,999,000	$ 28,634,000
State	3,365,000	1,034,000	2,298,000
Total current	29,457,000	22,033,000	30,932,000
Deferred:
Federal	1,577,000	4,022,000	(2,337,000)
State	43,000	319,000	390,000
Total deferred	1,620,000	4,341,000	(1,947,000)
Total provision	31,077,000	26,374,000	28,985,000
Reasons for the difference between income tax expense and statutory federal income tax
Statutory federal income tax	30,997,000	29,555,000	29,380,000
(Decrease) increase in income taxes resulting from Tax-exempt interest income	(1,152,000)	(1,236,000)	(1,219,000)
(Decrease) increase in income taxes resulting from state taxes, net of federal income tax benefit	2,215,000	2,300,000	1,747,000
(Decrease) increase in income taxes resulting from reduction in uncertain tax positions	0	(3,300,000)	0
(Decrease) increase in income taxes resulting from other reconciling items	(983,000)	(945,000)	(923,000)
Total provision	$ 31,077,000	$ 26,374,000	$ 28,985,000
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%
(Decrease) increase in income taxes resulting from Tax-exempt interest income (as a percent)	(1.30%)	(1.50%)	(1.50%)
(Decrease) increase in income taxes resulting from state taxes, net of federal income tax benefit (as a percent)	2.50%	2.70%	2.10%
(Decrease) increase in income taxes resulting from reduction in uncertain tax positions (as a percent)	0.00%	(3.90%)	0.00%
(Decrease) increase in income taxes resulting from other reconciling items (as a percent)	(1.10%)	(1.10%)	(1.10%)
Total (as a percent)	35.10%	31.20%	34.50%
Deferred tax assets:
Reserve for loan and lease losses	$ 34,410,000	$ 33,088,000
Accruals for employee benefits	3,816,000	3,346,000
Other	905,000	1,079,000
Total deferred tax assets	39,131,000	37,513,000
Deferred tax liabilities:
Differing depreciable bases in premises and leased equipment	27,274,000	24,506,000
Net unrealized gains on securities available-for-sale	3,940,000	5,654,000
Differing bases in assets related to acquisitions	5,738,000	5,160,000
Mortgage servicing	1,630,000	1,644,000
Capitalized loan costs	1,454,000	1,437,000
Prepaid expenses	1,055,000	1,035,000
Other	312,000	443,000
Total deferred tax liabilities	41,403,000	39,879,000
Net deferred tax liability	(2,272,000)	(2,366,000)
Reconciliation of the beginning and ending amounts of unrecognized tax benefits
Balance, at beginning of the period	0	4,611,000	$ 4,068,000
Additions based on tax positions related to the current year	380,000	66,000	484,000
Additions for tax positions of prior years	0	592,000	1,118,000
Reductions for tax positions of prior years	0	(553,000)	0
Reductions due to lapse in statute of limitations	0	(1,650,000)	(1,059,000)
Settlements	0	(3,066,000)	0
Balance at end of the period	380,000	0	4,611,000
Valuation allowance for deferred tax assets	0	0
Unrecognized tax benefits that would affect the effective tax rate if recognized	250,000.00	0	2,620,000
Interest and penalties net of tax recognized	0	(690,000)	140,000
Accrued interest and penalties	0	0	690,000
Increase in earnings from settlement of unrecognized tax benefits		2,120,000
Investment securities available-for-sale
Deferred:
Total provision	2,000	361,000	(63,000)
Reasons for the difference between income tax expense and statutory federal income tax
Total provision	$ 2,000	361,000	$ (63,000)
State and local jurisdiction
Reconciliation of the beginning and ending amounts of unrecognized tax benefits
Settlements		$ (2,930,000)