Fair Value Measurements - Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Financial liabilities:
Preferred stock pending redemption	$ 50,000	$ 0
Carrying Amount
Financial assets:
Cash and cash equivalents	801,332	472,447
Investment securities available-for-sale	1,995,258	2,016,864
Investment securities held-to-maturity	208,223	152,781
Accrued interest receivable	28,869	31,974
Mortgage servicing rights	12,653	11,555
Net loans	4,123,401	4,073,968
Total financial assets	7,169,736	6,759,589
Financial liabilities:
Total deposits, excluding time deposits	4,854,927	4,269,631
Time deposits	1,385,484	1,557,340
Securities sold under repurchase agreements	505,785	516,243
Other borrowed funds	32	7
Accrued interest payable	6,502	8,123
Long-term debt	37,160	37,200
Preferred stock pending redemption	50,000
Subordinated debentures held by subsidiary trusts	82,477	123,715
Total financial liabilities	6,922,367	6,512,642
Estimated Fair Value
Financial assets:
Cash and cash equivalents	801,332	472,447
Investment securities available-for-sale	1,995,258	2,016,864
Investment securities held-to-maturity	218,933	161,877
Accrued interest receivable	28,869	31,974
Mortgage servicing rights	16,373	11,910
Net loans	4,142,426	4,064,718
Total financial assets	7,203,191	6,759,790
Financial liabilities:
Total deposits, excluding time deposits	4,854,927	4,269,631
Time deposits	1,394,403	1,565,558
Securities sold under repurchase agreements	505,785	516,243
Other borrowed funds	32	7
Accrued interest payable	6,502	8,123
Long-term debt	35,104	34,341
Preferred stock pending redemption	50,000
Subordinated debentures held by subsidiary trusts	62,409	102,525
Total financial liabilities	6,909,162	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
Preferred stock pending redemption	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	801,332	472,447
Investment securities available-for-sale	1,995,258	2,016,864
Investment securities held-to-maturity	218,933	161,877
Accrued interest receivable	28,869	31,974
Mortgage servicing rights	16,373	11,910
Net loans	4,067,803	3,964,683
Total financial assets	7,128,568	6,659,755
Financial liabilities:
Total deposits, excluding time deposits	4,854,927	4,269,631
Time deposits	1,394,403	1,565,558
Securities sold under repurchase agreements	505,785	516,243
Other borrowed funds	32	7
Accrued interest payable	6,502	8,123
Long-term debt	35,104	34,341
Preferred stock pending redemption	50,000
Subordinated debentures held by subsidiary trusts	62,409	102,525
Total financial liabilities	6,909,162	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
Net loans		100,035
Total financial assets	74,623	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
Preferred stock pending redemption	0
Subordinated debentures held by subsidiary trusts		0
Total financial liabilities	$ 0	$ 383