Fair Value Measurements - Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Financial assets:
Investment securities held-to-maturity	$ 205,926	$ 218,933
Financial liabilities:
Preferred stock pending redemption	0	50,000
Carrying Amount
Financial assets:
Cash and cash equivalents	534,827	801,332
Investment securities available-for-sale	1,947,706	1,995,258
Investment securities held-to-maturity	203,837	208,223
Accrued interest receivable	26,450	28,869
Mortgage servicing rights	13,546	12,653
Net loans	4,259,514	4,123,401
Total financial assets	6,985,880	7,169,736
Financial liabilities:
Total deposits, excluding time deposits	4,943,033	4,854,927
Time deposits	1,190,717	1,385,484
Securities sold under repurchase agreements	457,437	505,785
Other borrowed funds	3	32
Accrued interest payable	4,963	6,502
Long-term debt	36,917	37,160
Preferred stock pending redemption		50,000
Subordinated debentures held by subsidiary trusts	82,477	82,477
Total financial liabilities	6,715,547	6,922,367
Estimated Fair Value
Financial assets:
Cash and cash equivalents	534,827	801,332
Investment securities available-for-sale	1,947,706	1,995,258
Investment securities held-to-maturity	205,926	218,933
Accrued interest receivable	26,450	28,869
Mortgage servicing rights	25,698	16,373
Net loans	4,246,539	4,142,426
Total financial assets	6,987,146	7,203,191
Financial liabilities:
Total deposits, excluding time deposits	4,943,033	4,854,927
Time deposits	1,196,250	1,394,403
Securities sold under repurchase agreements	457,437	505,785
Other borrowed funds	3	32
Accrued interest payable	4,963	6,502
Long-term debt	34,508	35,104
Preferred stock pending redemption		50,000
Subordinated debentures held by subsidiary trusts	72,045	62,409
Total financial liabilities	6,708,239	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
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	534,827	801,332
Investment securities available-for-sale	1,947,706	1,995,258
Investment securities held-to-maturity	205,926	218,933
Accrued interest receivable	26,450	28,869
Mortgage servicing rights	25,698	16,373
Net loans	4,189,237	4,067,803
Total financial assets	6,929,844	7,128,568
Financial liabilities:
Total deposits, excluding time deposits	4,943,033	4,854,927
Time deposits	1,196,250	1,394,403
Securities sold under repurchase agreements	457,437	505,785
Other borrowed funds	3	32
Accrued interest payable	4,963	6,502
Long-term debt	34,508	35,104
Preferred stock pending redemption		50,000
Subordinated debentures held by subsidiary trusts	72,045	62,409
Total financial liabilities	6,708,239	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	57,302	74,623
Total financial assets	57,302	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
Preferred stock pending redemption		0
Subordinated debentures held by subsidiary trusts	0	0
Total financial liabilities	$ 0	$ 0