Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Financial liabilities:
Preferred Stock, Pending Redemption	$ 0	$ 50,000
Carrying Amount
Financial assets:
Cash and cash equivalents	498,543	801,332
Investment securities available-for-sale	2,011,169	1,995,258
Investment securities held-to-maturity	210,426	208,223
Accrued interest receivable	29,375	28,869
Mortgage servicing rights	13,006	12,653
Net loans	4,126,655	4,123,401
Total financial assets	6,889,174	7,169,736
Financial liabilities:
Total deposits, excluding time deposits	4,684,664	4,854,927
Time deposits	1,343,681	1,385,484
Securities sold under repurchase agreements	467,205	505,785
Other borrowed funds	8	32
Accrued interest payable	6,433	6,502
Long-term debt	37,150	37,160
Preferred Stock, Pending Redemption		50,000
Subordinated debentures held by subsidiary trusts	82,477	82,477
Total financial liabilities	6,621,618	6,922,367
Estimated Fair Value
Financial assets:
Cash and cash equivalents	498,543	801,332
Investment securities available-for-sale	2,011,169	1,995,258
Investment securities held-to-maturity	220,115	218,933
Accrued interest receivable	29,375	28,869
Mortgage servicing rights	18,807	16,373
Net loans	4,139,777	4,142,426
Total financial assets	6,917,786	7,203,191
Financial liabilities:
Total deposits, excluding time deposits	4,684,664	4,854,927
Time deposits	1,351,742	1,394,403
Securities sold under repurchase agreements	467,205	505,785
Other borrowed funds	8	32
Accrued interest payable	6,433	6,502
Long-term debt	34,887	35,104
Preferred Stock, Pending Redemption		50,000
Subordinated debentures held by subsidiary trusts	66,080	62,409
Total financial liabilities	6,611,019	6,909,162
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
Total financial liabilities	0	0
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	498,543	801,332
Investment securities available-for-sale	2,011,169	1,995,258
Investment securities held-to-maturity	220,115	218,933
Accrued interest receivable	29,375	28,869
Mortgage servicing rights	18,807	16,373
Net loans	4,062,688	4,067,803
Total financial assets	6,840,697	7,128,568
Financial liabilities:
Total deposits, excluding time deposits	4,684,664	4,854,927
Time deposits	1,351,742	1,394,403
Securities sold under repurchase agreements	467,205	505,785
Other borrowed funds	8	32
Accrued interest payable	6,433	6,502
Long-term debt	34,887	35,104
Preferred Stock, Pending Redemption		50,000
Subordinated debentures held by subsidiary trusts	66,080	62,409
Total financial liabilities	6,611,019	6,909,162
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	77,089	74,623
Total financial assets	77,089	74,623
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
Total financial liabilities	$ 0	$ 0