Income Taxes (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Current [Abstract]
Federal	$ 46,749	$ 43,806	$ 42,117
State	4,712	4,702	6,393
Deferred [Abstract]
Federal	(8,414)	(5,558)	(3,892)
State	(1,269)	(831)	(585)
Income tax provision	41,778	42,119	44,033
Reconciliation of provision for tax from the federal rate to the effective tax rate [Abstract]
Income tax computed at statutory tax rate	52,017	49,964	47,978
Tax exempt interest	(5,244)	(5,115)	(5,066)
Nondeductible interest expense	153	181	270
State income taxes, net	2,238	2,517	3,775
Income tax credits	(7,633)	(6,729)	(3,396)
Other	247	1,301	472
Income tax provision	41,778	42,119	44,033
Statutory federal income tax rate	35.00%	35.00%	35.00%
Deferred tax assets [Abstract]
Allowance for loan losses	34,433	35,767
Pension and other postretirement benefit plans	31,606	24,271
Other real estate	25,113	22,048
Stock-based compensation	5,387	6,249
Deferred compensation	4,519	4,444
Other	11,094	6,102
Gross deferred tax asset	112,152	98,881
Deferred tax liabilities [Abstract]
Unrealized gains on securities available for sale	28,202	13,072
Goodwill and other identifiable intangibles	15,871	15,478
Premises and equipment	15,336	13,185
Mortgage servicing rights	2,925	6,019
Securities	2,167	5,034
Other	3,485	2,942
Gross deferred tax liability	67,986	55,730
Net deferred tax asset	44,166	43,151
Changes in unrecognized tax benefits [Roll Forward]
Balance at beginning of period	1,397
Increases due to tax positions taken during the current year	208
Decreases due to tax positions taken during a prior year	(123)
Decreases due to the lapse of applicable statute of limitations during the current year	(318)
Balance at end of period	1,164	1,397
Accrued interest, net of federal benefit, at end of period	372
Unrecognized tax benefits that would impact the effective tax rate, if recognized, at December 31, 2011	$ 814