Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Financial assets:
Investment securities held-to-maturity	$ 593,904	$ 607,550
Carrying Amount
Financial assets:
Cash and cash equivalents	655,528	780,457
Investment securities available-for-sale	1,563,350	1,456,840
Investment securities held-to-maturity	581,390	600,665
Accrued interest receivable	26,907	27,729
Mortgage servicing rights	15,574	15,621
Net loans	5,164,534	5,169,379
Derivative Asset	2,138	953
Deferred compensation asset at fair value amount	9,566	10,149
Total financial assets	8,018,987	8,061,793
Financial liabilities:
Total deposits, excluding time deposits	6,004,127	5,957,345
Time deposits	1,103,336	1,131,592
Securities sold under repurchase agreements	465,523	510,635
Other borrowed funds	33	2
Derivative Liability	3,619	829
Deferred compensation plan liabilities at fair value amount	9,566	10,149
Accrued interest payable	5,184	4,960
Long-term debt	27,907	27,885
Subordinated debentures held by subsidiary trusts	82,477	82,477
Total financial liabilities	7,701,772	7,725,874
Estimated Fair Value
Financial assets:
Cash and cash equivalents	655,528	780,457
Investment securities available-for-sale	1,563,350	1,456,840
Investment securities held-to-maturity	593,904	607,550
Accrued interest receivable	26,907	27,729
Mortgage servicing rights	26,359	31,011
Net loans	5,105,350	5,128,705
Derivative Asset	2,138	953
Deferred compensation asset at fair value amount	9,566	10,149
Total financial assets	7,983,102	8,043,394
Financial liabilities:
Total deposits, excluding time deposits	6,004,127	5,975,345
Time deposits	1,100,852	1,137,289
Securities sold under repurchase agreements	465,523	510,635
Other borrowed funds	33	2
Derivative Liability	3,619	829
Deferred compensation plan liabilities at fair value amount	9,566	10,149
Accrued interest payable	5,184	4,960
Long-term debt	27,662	27,622
Subordinated debentures held by subsidiary trusts	72,252	74,969
Total financial liabilities	7,688,818	7,741,800
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	655,528	780,457
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
Derivative Asset	0	0
Deferred compensation asset at fair value amount	0	0
Total financial assets	655,528	780,457
Financial liabilities:
Total deposits, excluding time deposits	6,004,127	5,975,345
Time deposits	0	0
Securities sold under repurchase agreements	0	0
Other borrowed funds	0	0
Derivative Liability	0	0
Deferred compensation plan liabilities at fair value amount	0	0
Accrued interest payable	0	0
Long-term debt	0	0
Subordinated debentures held by subsidiary trusts	0	0
Total financial liabilities	6,004,127	5,975,345
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment securities available-for-sale	1,563,350	1,456,840
Investment securities held-to-maturity	593,904	607,550
Accrued interest receivable	26,907	27,729
Mortgage servicing rights	26,359	31,011
Net loans	5,085,987	5,107,830
Derivative Asset	2,138	953
Deferred compensation asset at fair value amount	9,566	10,149
Total financial assets	7,308,211	7,242,062
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	1,100,852	1,137,289
Securities sold under repurchase agreements	465,523	510,635
Other borrowed funds	33	2
Derivative Liability	3,619	829
Deferred compensation plan liabilities at fair value amount	9,566	10,149
Accrued interest payable	5,184	4,960
Long-term debt	27,662	27,622
Subordinated debentures held by subsidiary trusts	72,252	74,969
Total financial liabilities	1,684,691	1,766,455
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	19,363	20,875
Derivative Asset	0	0
Deferred compensation asset at fair value amount	0	0
Total financial assets	19,363	20,875
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	0
Deferred compensation plan liabilities at fair value amount	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