Income Taxes (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current [Abstract]
Federal	$ 21,011,000	$ 29,274,000	$ 34,124,000
State	1,815,000	1,477,000	1,743,000
Current tax expense (benefit)	22,826,000	30,751,000	35,867,000
Deferred [Abstract]
Federal	(13,000)	(8,129,000)	(12,121,000)
State	3,000	(1,349,000)	(2,834,000)
Deferred tax expense (benefit)	(10,000)	(9,478,000)	(14,955,000)
Total income tax expense	22,816,000	21,273,000	20,912,000
Income tax recorded to stockholders' equity	(100,000)	(500,000)	(4,300,000)
Deferred tax assets [Abstract]
Allowance for loan and lease losses	26,687,000	27,487,000
Deferred compensation	5,362,000	4,911,000
Postretirement benefit obligation	1,600,000	1,036,000
Fair value adjustments from acquisitions	3,633,000	2,227,000
Accrued liabilities	1,524,000	1,482,000
Stock-based compensation expense	5,726,000	4,440,000
Other	1,933,000	1,214,000
Total deferred tax assets	46,465,000	42,797,000
Deferred tax liabilities [Abstract]
Pension and executive retirement	7,676,000	6,916,000
Unrealized gains on securities available for sale	10,939,000	12,156,000
Premises and equipment, primarily due to accelerated depreciation	1,980,000	2,340,000
Equipment leasing	582,000	3,090,000
Deferred loan costs	873,000	767,000
Intangible amortization	13,146,000	11,081,000
Other	154,000	494,000
Total deferred tax liabilities	35,350,000	36,844,000
Net deferred tax asset at year-end	11,115,000	5,953,000	(4,023,000)
Net deferred tax asset (liability) at beginning of year	5,953,000	(4,023,000)
Increase in net deferred tax asset	5,162,000	9,976,000
Reconciliation of gross unrecognized tax benefits [Roll Forward]
Balance, beginning of period	888,000	3,081,000	2,287,000
Additions for tax positions of prior years	0	0	1,714,000
Reduction for tax positions of prior years	(888,000)	(2,193,000)	(920,000)
Balance, end of period	0	888,000	3,081,000
Decrease in tax expense resulting from lapse of statute of limitations	800,000	1,500,000
Accrued interest		100,000
Net interest		300,000	300,000
Federal statutory income tax rate (in hundredths)	35.00%
Income tax reconciliation [Abstract]
Federal income tax at statutory rate	27,081,000	27,711,000	27,410,000
Tax exempt income	(2,536,000)	(2,925,000)	(3,448,000)
Net increase in CSV of life insurance	(908,000)	(919,000)	(939,000)
Low income housing tax credits	(629,000)	(782,000)	(296,000)
State taxes, net of federal tax benefit	1,182,000	764,000	756,000
State audit settlements	0	(681,000)	(1,465,000)
Other, net	(1,374,000)	(1,895,000)	(1,106,000)
Income tax expense	$ 22,816,000	$ 21,273,000	$ 20,912,000