Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financial assets:
Investment securities held-to-maturity	$ 534,633	$ 584,533
Carrying Amount
Financial assets:
Cash and cash equivalents	708,295	798,670
Investment securities available-for-sale	1,545,093	1,711,924
Investment securities held-to-maturity	522,543	575,186
Accrued interest receivable	31,590	27,063
Mortgage servicing rights	15,336	14,038
Net loans	5,102,244	4,823,243
Total financial assets	7,925,101	7,950,124
Financial liabilities:
Total deposits, excluding time deposits	5,885,462	5,767,992
Time deposits	1,150,332	1,238,220
Securities sold under repurchase agreements	437,533	502,250
Other borrowed funds	10	9
Derivative Liability	436
Accrued interest payable	5,327	5,833
Long-term debt	43,089	38,067
Subordinated debentures held by subsidiary trusts	82,477	82,477
Total financial liabilities	7,604,666	7,634,848
Estimated Fair Value
Financial assets:
Cash and cash equivalents	708,295	798,670
Investment securities available-for-sale	1,545,093	1,711,924
Investment securities held-to-maturity	534,633	584,533
Accrued interest receivable	31,590	27,063
Mortgage servicing rights	31,544	21,434
Net loans	5,055,788	4,800,725
Total financial assets	7,906,943	7,944,349
Financial liabilities:
Total deposits, excluding time deposits	5,885,462	5,767,992
Time deposits	1,154,128	1,244,324
Securities sold under repurchase agreements	437,533	502,250
Other borrowed funds	10	9
Derivative Liability	436
Accrued interest payable	5,327	5,833
Long-term debt	41,818	37,781
Subordinated debentures held by subsidiary trusts	74,723	75,734
Total financial liabilities	7,599,437	7,633,923
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	708,295	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	708,295	798,670
Financial liabilities:
Total deposits, excluding time deposits	5,885,462	5,767,992
Time deposits	0	0
Securities sold under repurchase agreements	0	0
Other borrowed funds	0	0
Derivative Liability	0
Accrued interest payable	0	0
Long-term debt	0	0
Subordinated debentures held by subsidiary trusts	0	0
Total financial liabilities	5,885,462	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	1,545,093	1,711,924
Investment securities held-to-maturity	534,633	584,533
Accrued interest receivable	31,590	27,063
Mortgage servicing rights	31,544	21,434
Net loans	5,010,848	4,770,231
Total financial assets	7,153,708	7,115,185
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	1,154,128	1,244,324
Securities sold under repurchase agreements	437,533	502,250
Other borrowed funds	10	9
Derivative Liability	436
Accrued interest payable	5,327	5,833
Long-term debt	41,818	37,781
Subordinated debentures held by subsidiary trusts	74,723	75,734
Total financial liabilities	1,713,975	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	44,940	30,494
Total financial assets	44,940	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
Derivative Liability	0
Accrued interest payable	0	0
Long-term debt	0	0
Subordinated debentures held by subsidiary trusts	0	0
Total financial liabilities	$ 0	$ 0