Income Taxes (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current [Abstract]
Federal	$ 26,059,000	$ 23,536,000	$ 21,011,000
State	2,823,000	2,316,000	1,815,000
Current tax expense (benefit)	28,882,000	25,852,000	22,826,000
Deferred [Abstract]
Federal	6,648,000	2,334,000	(13,000)
State	1,699,000	10,000	3,000
Deferred tax expense (benefit)	8,347,000	2,344,000	(10,000)
Total income tax expense	37,229,000	28,196,000	22,816,000
Income tax recorded to stockholders' equity	(1,100,000)	7,000,000	(100,000)
Deferred tax assets [Abstract]
Allowance for loan and lease losses	25,689,000	26,729,000
Deferred compensation	8,744,000	7,931,000
Postretirement benefit obligation	2,377,000	2,596,000
Unrealized losses on securities available for sale	0	5,900,000
Accrued liabilities	1,778,000	3,265,000
Stock-based compensation expense	6,457,000	7,862,000
Equipment leasing	1,416,000	1,661,000
Other	1,325,000	2,125,000
Total deferred tax assets	47,786,000	58,069,000
Deferred tax liabilities [Abstract]
Pension and executive retirement	12,192,000	17,417,000
Fair value adjustments from acquisitions	1,243,000	2,070,000
Unrealized gains on securities available for sale	1,331,000	0
Premises and equipment, primarily due to accelerated depreciation	2,491,000	2,270,000
Deferred loan costs	1,460,000	1,624,000
Intangible amortization	14,162,000	12,007,000
Other	390,000	892,000
Total deferred tax liabilities	33,269,000	36,280,000
Net deferred tax asset at year-end	14,517,000	21,789,000	11,115,000
Net deferred tax asset (liability) at beginning of year	21,789,000	11,115,000
Increase in net deferred tax asset	(7,272,000)	10,674,000
Reduction in unrecognized tax benefits resulting from expiration of statute of limitations of prior years' tax filings			(900,000)
Decrease in tax expense			(800,000)
Federal statutory income tax rate (in hundredths)	35.00%
Income tax reconciliation [Abstract]
Federal income tax at statutory rate	39,306,000	31,482,000	27,081,000
Tax exempt income	(2,250,000)	(2,433,000)	(2,536,000)
Net increase in CSV of life insurance	(1,734,000)	(1,166,000)	(908,000)
Low income housing tax credits	(880,000)	(819,000)	(629,000)
State taxes, net of federal tax benefit	2,939,000	1,512,000	1,182,000
Other, net	(152,000)	(380,000)	(1,374,000)
Total income tax expense	$ 37,229,000	$ 28,196,000	$ 22,816,000