Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Financial assets:
Investment securities held-to-maturity	$ 576,931	$ 584,533
Carrying Amount
Financial assets:
Cash and cash equivalents	637,803	798,670
Investment securities available-for-sale	1,777,057	1,711,924
Investment securities held-to-maturity	563,847	575,186
Accrued interest receivable	27,242	27,063
Mortgage servicing rights	14,093	14,038
Net loans	4,851,970	4,823,243
Total financial assets	7,872,012	7,950,124
Financial liabilities:
Total deposits, excluding time deposits	5,763,553	5,767,992
Time deposits	1,205,025	1,238,220
Securities sold under repurchase agreements	462,073	502,250
Other borrowed funds	4	9
Accrued interest payable	5,113	5,833
Long-term debt	43,048	38,067
Subordinated debentures held by subsidiary trusts	82,477	82,477
Total financial liabilities	7,561,293	7,634,848
Estimated Fair Value
Financial assets:
Cash and cash equivalents	637,803	798,670
Investment securities available-for-sale	1,777,057	1,711,924
Investment securities held-to-maturity	576,930	584,533
Accrued interest receivable	27,242	27,063
Mortgage servicing rights	26,639	21,434
Net loans	4,821,593	4,800,725
Total financial assets	7,867,264	7,944,349
Financial liabilities:
Total deposits, excluding time deposits	5,763,553	5,767,992
Time deposits	1,211,057	1,244,324
Securities sold under repurchase agreements	462,073	502,250
Other borrowed funds	4	9
Accrued interest payable	5,113	5,833
Long-term debt	38,047	37,781
Subordinated debentures held by subsidiary trusts	75,679	75,734
Total financial liabilities	7,555,526	7,633,923
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	637,803	798,670
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	637,803	798,670
Financial liabilities:
Total deposits, excluding time deposits	0	5,767,992
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	5,767,992
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment securities available-for-sale	1,777,057	1,711,924
Investment securities held-to-maturity	576,930	584,533
Accrued interest receivable	27,242	27,063
Mortgage servicing rights	26,639	21,434
Net loans	4,786,495	4,770,231
Total financial assets	7,194,363	7,115,185
Financial liabilities:
Total deposits, excluding time deposits	5,763,553	0
Time deposits	1,211,057	1,244,324
Securities sold under repurchase agreements	462,073	502,250
Other borrowed funds	4	9
Accrued interest payable	5,113	5,833
Long-term debt	38,047	37,781
Subordinated debentures held by subsidiary trusts	75,679	75,734
Total financial liabilities	7,555,526	1,865,931
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	35,098	30,494
Total financial assets	35,098	30,494
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