Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Carrying Amount
Financial assets:
Cash and cash equivalents	$ 536,653	$ 472,447
Investment securities available-for-sale	1,913,983	2,016,864
Investment securities held-to-maturity	166,926	152,781
Accrued interest receivable	30,936	31,974
Mortgage servicing rights	11,985	11,555
Net loans	4,067,169	4,073,968
Total financial assets	6,727,652	6,759,589
Financial liabilities:
Total deposits, excluding time deposits	4,419,969	4,269,631
Time deposits	1,481,410	1,557,340
Securities sold under repurchase agreements	455,993	516,243
Other borrowed funds	7	7
Accrued interest payable	8,215	8,123
Long-term debt	37,181	37,200
Subordinated debentures held by subsidiary trusts	82,477	123,715
Derivative liability contract		383
Total financial liabilities	6,485,252	6,512,642
Estimated Fair Value
Financial assets:
Cash and cash equivalents	536,653	472,447
Investment securities available-for-sale	1,913,983	2,016,864
Investment securities held-to-maturity	177,532	161,877
Accrued interest receivable	30,936	31,974
Mortgage servicing rights	14,180	11,910
Net loans	4,055,520	4,064,718
Total financial assets	6,728,804	6,759,790
Financial liabilities:
Total deposits, excluding time deposits	4,419,969	4,269,631
Time deposits	1,488,737	1,565,558
Securities sold under repurchase agreements	455,993	516,243
Other borrowed funds	7	7
Accrued interest payable	8,215	8,123
Long-term debt	34,101	34,341
Subordinated debentures held by subsidiary trusts	61,380	102,525
Derivative liability contract		383
Total financial liabilities	6,468,402	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	536,653	472,447
Investment securities available-for-sale	1,913,983	2,016,864
Investment securities held-to-maturity	177,532	161,877
Accrued interest receivable	30,936	31,974
Mortgage servicing rights	14,180	11,910
Net loans	3,985,627	3,964,683
Total financial assets	6,658,911	6,659,755
Financial liabilities:
Total deposits, excluding time deposits	4,419,969	4,269,631
Time deposits	1,488,737	1,565,558
Securities sold under repurchase agreements	455,993	516,243
Other borrowed funds	7	7
Accrued interest payable	8,215	8,123
Long-term debt	34,101	34,341
Subordinated debentures held by subsidiary trusts	61,380	102,525
Derivative liability contract		0
Total financial liabilities	6,468,402	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	69,893	100,035
Total financial assets	69,893	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