Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Carrying Amount
Financial assets:
Cash and cash equivalents	$ 611,335	$ 472,447
Investment securities available-for-sale	1,979,154	2,016,864
Investment securities held-to-maturity	187,573	152,781
Accrued interest receivable	33,416	31,974
Mortgage servicing rights	12,334	11,555
Net loans	4,081,045	4,073,968
Total financial assets	6,904,857	6,759,589
Financial liabilities:
Total deposits, excluding time deposits	4,612,346	4,269,631
Time deposits	1,423,386	1,557,340
Securities sold under repurchase agreements	460,805	516,243
Other borrowed funds	6	7
Accrued interest payable	6,706	8,123
Long-term debt	37,170	37,200
Subordinated debentures held by subsidiary trusts	82,477	123,715
Derivative liability contract	6,622,896	383
Total financial liabilities		6,512,642
Estimated Fair Value
Financial assets:
Cash and cash equivalents	611,335	472,447
Investment securities available-for-sale	1,979,154	2,016,864
Investment securities held-to-maturity	199,078	161,877
Accrued interest receivable	33,416	31,974
Mortgage servicing rights	14,082	11,910
Net loans	4,067,457	4,064,718
Total financial assets	6,904,522	6,759,790
Financial liabilities:
Total deposits, excluding time deposits	4,612,346	4,269,631
Time deposits	1,431,066	1,565,558
Securities sold under repurchase agreements	460,805	516,243
Other borrowed funds	6	7
Accrued interest payable	6,706	8,123
Long-term debt	34,003	34,341
Subordinated debentures held by subsidiary trusts	57,656	102,525
Derivative liability contract		383
Total financial liabilities	6,602,588	6,496,811
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment securities available-for-sale	0	0
Investment securities held-to-maturity	0	0
Accrued interest receivable	0	0
Mortgage servicing rights	0	0
Net loans	0	0
Total financial assets	0	0
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	0	0
Securities sold under repurchase agreements	0	0
Other borrowed funds	0	0
Accrued interest payable	0	0
Long-term debt	0	0
Subordinated debentures held by subsidiary trusts	0	0
Derivative liability contract		0
Total financial liabilities	0	0
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	611,335	472,447
Investment securities available-for-sale	1,979,154	2,016,864
Investment securities held-to-maturity	199,078	161,877
Accrued interest receivable	33,416	31,974
Mortgage servicing rights	14,082	11,910
Net loans	4,000,902	3,964,683
Total financial assets	6,837,967	6,659,755
Financial liabilities:
Total deposits, excluding time deposits	4,612,346	4,269,631
Time deposits	1,431,066	1,565,558
Securities sold under repurchase agreements	460,805	516,243
Other borrowed funds	6	7
Accrued interest payable	6,706	8,123
Long-term debt	34,003	34,341
Subordinated debentures held by subsidiary trusts	57,656	102,525
Derivative liability contract		0
Total financial liabilities	6,602,588	6,496,428
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment securities available-for-sale	0	0
Investment securities held-to-maturity	0	0
Accrued interest receivable	0	0
Mortgage servicing rights	0	0
Net loans	66,555	100,035
Total financial assets	66,555	100,035
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	0	0
Securities sold under repurchase agreements	0	0
Other borrowed funds	0	0
Accrued interest payable	0	0
Long-term debt	0	0
Subordinated debentures held by subsidiary trusts	0	0
Derivative liability contract		383
Total financial liabilities	$ 0	$ 383