Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Financial assets:
Investment securities held-to-maturity	$ 205,443	$ 205,926
Carrying Amount
Financial assets:
Cash and cash equivalents	610,531	534,827
Investment securities available-for-sale	1,893,652	1,947,706
Investment securities held-to-maturity	201,436	203,837
Accrued interest receivable	27,135	26,450
Mortgage servicing rights	13,474	13,546
Net loans	4,283,467	4,259,514
Total financial assets	7,029,695	6,985,880
Financial liabilities:
Total deposits, excluding time deposits	4,975,781	4,943,033
Time deposits	1,159,356	1,190,717
Securities sold under repurchase agreements	488,898	457,437
Other borrowed funds	9	3
Accrued interest payable	4,953	4,963
Long-term debt	36,905	36,917
Subordinated debentures held by subsidiary trusts	82,477	82,477
Total financial liabilities	6,748,379	6,715,547
Estimated Fair Value
Financial assets:
Cash and cash equivalents	610,531	534,827
Investment securities available-for-sale	1,893,652	1,947,706
Investment securities held-to-maturity	205,443	205,926
Accrued interest receivable	27,135	26,450
Mortgage servicing rights	26,215	25,698
Net loans	4,272,639	4,246,539
Total financial assets	7,035,615	6,987,146
Financial liabilities:
Total deposits, excluding time deposits	4,975,781	4,943,033
Time deposits	1,166,103	1,196,250
Securities sold under repurchase agreements	488,898	457,437
Other borrowed funds	9	3
Accrued interest payable	4,953	4,963
Long-term debt	35,200	34,508
Subordinated debentures held by subsidiary trusts	73,607	72,045
Total financial liabilities	6,744,551	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	610,531	534,827
Investment securities available-for-sale	1,893,652	1,947,706
Investment securities held-to-maturity	205,443	205,926
Accrued interest receivable	27,135	26,450
Mortgage servicing rights	26,215	25,698
Net loans	4,224,292	4,189,237
Total financial assets	6,987,268	6,929,844
Financial liabilities:
Total deposits, excluding time deposits	4,975,781	4,943,033
Time deposits	1,166,103	1,196,250
Securities sold under repurchase agreements	488,898	457,437
Other borrowed funds	9	3
Accrued interest payable	4,953	4,963
Long-term debt	35,200	34,508
Subordinated debentures held by subsidiary trusts	73,607	72,045
Total financial liabilities	6,744,551	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	48,347	57,302
Total financial assets	48,347	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