Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Financial assets:
Investment securities held-to-maturity	$ 592,682	$ 205,926
Carrying Amount
Financial assets:
Cash and cash equivalents	503,648	534,827
Investment securities available-for-sale	1,506,996	1,947,706
Investment securities held-to-maturity	586,989	203,837
Accrued interest receivable	26,497	26,450
Mortgage servicing rights	13,443	13,546
Net loans	4,428,096	4,259,514
Total financial assets	7,065,669	6,985,880
Financial liabilities:
Total deposits, excluding time deposits	5,043,387	4,943,033
Time deposits	1,135,655	1,190,717
Securities sold under repurchase agreements	462,985	457,437
Other borrowed funds	20	3
Accrued interest payable	5,316	4,963
Long-term debt	36,893	36,917
Subordinated debentures held by subsidiary trusts	82,477	82,477
Total financial liabilities	6,766,733	6,715,547
Estimated Fair Value
Financial assets:
Cash and cash equivalents	503,648	534,827
Investment securities available-for-sale	1,506,996	1,947,706
Investment securities held-to-maturity	592,682	205,926
Accrued interest receivable	26,497	26,450
Mortgage servicing rights	25,812	25,698
Net loans	4,389,772	4,246,539
Total financial assets	7,045,407	6,987,146
Financial liabilities:
Total deposits, excluding time deposits	5,043,387	4,943,033
Time deposits	1,141,585	1,196,250
Securities sold under repurchase agreements	462,985	457,437
Other borrowed funds	20	3
Accrued interest payable	5,316	4,963
Long-term debt	35,381	34,508
Subordinated debentures held by subsidiary trusts	75,200	72,045
Total financial liabilities	6,763,874	6,708,239
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	503,648	534,827
Investment securities available-for-sale	1,506,996	1,947,706
Investment securities held-to-maturity	592,682	205,926
Accrued interest receivable	26,497	26,450
Mortgage servicing rights	25,812	25,698
Net loans	4,343,267	4,189,237
Total financial assets	6,998,902	6,929,844
Financial liabilities:
Total deposits, excluding time deposits	5,043,387	4,943,033
Time deposits	1,141,585	1,196,250
Securities sold under repurchase agreements	462,985	457,437
Other borrowed funds	20	3
Accrued interest payable	5,316	4,963
Long-term debt	35,381	34,508
Subordinated debentures held by subsidiary trusts	75,200	72,045
Total financial liabilities	6,763,874	6,708,239
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	46,505	57,302
Total financial assets	46,505	57,302
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