Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Financial assets:
Investment securities held-to-maturity	$ 574,482	$ 607,550
Carrying Amount
Financial assets:
Cash and cash equivalents	476,051	780,457
Investment securities available-for-sale	1,505,581	1,456,840
Investment securities held-to-maturity	566,247	600,665
Accrued interest receivable	27,448	27,729
Mortgage servicing rights	16,038	15,621
Mortgages Held-for-sale, Fair Value Disclosure	73,053
Loans Receivable, Fair Value Disclosure	5,259,849	5,169,379
Derivative Asset	4,520	953
Deferred compensation asset at fair value amount	10,247	10,149
Total financial assets	7,939,034	8,061,793
Financial liabilities:
Total deposits, excluding time deposits	5,894,902	5,957,345
Time deposits	1,086,546	1,131,592
Securities sold under repurchase agreements	466,399	510,635
Other borrowed funds	15	2
Accrued interest payable	5,647	4,960
Long-term debt	27,928	27,885
Subordinated debentures held by subsidiary trusts	82,477	82,477
Derivative Liability	6,260	829
Deferred compensation plan liabilities at fair value amount	10,247	10,149
Total financial liabilities	7,580,421	7,725,874
Estimated Fair Value
Financial assets:
Cash and cash equivalents	476,051	780,457
Investment securities available-for-sale	1,505,581	1,456,840
Investment securities held-to-maturity	574,482	607,550
Accrued interest receivable	27,448	27,729
Mortgage servicing rights	23,970	31,011
Mortgages Held-for-sale, Fair Value Disclosure	73,053
Loans Receivable, Fair Value Disclosure	5,123,545	5,128,705
Derivative Asset	4,520	953
Deferred compensation asset at fair value amount	10,247	10,149
Total financial assets	7,818,897	8,043,394
Financial liabilities:
Total deposits, excluding time deposits	5,894,902	5,975,345
Time deposits	1,085,410	1,137,289
Securities sold under repurchase agreements	466,399	510,635
Other borrowed funds	15	2
Accrued interest payable	5,647	4,960
Long-term debt	27,535	27,622
Subordinated debentures held by subsidiary trusts	71,173	74,969
Derivative Liability	6,260	829
Deferred compensation plan liabilities at fair value amount	10,247	10,149
Total financial liabilities	7,567,588	7,741,800
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	476,051	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
Mortgages Held-for-sale, Fair Value Disclosure	0
Loans Receivable, Fair Value Disclosure	0	0
Derivative Asset	0	0
Deferred compensation asset at fair value amount	0	0
Total financial assets	476,051	780,457
Financial liabilities:
Total deposits, excluding time deposits	5,894,902	5,975,345
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
Derivative Liability	0	0
Deferred compensation plan liabilities at fair value amount	0	0
Total financial liabilities	5,894,902	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	1,505,581	1,456,840
Investment securities held-to-maturity	574,482	607,550
Accrued interest receivable	27,448	27,729
Mortgage servicing rights	23,970	31,011
Mortgages Held-for-sale, Fair Value Disclosure	73,053
Loans Receivable, Fair Value Disclosure	5,102,007	5,107,830
Derivative Asset	4,520	953
Deferred compensation asset at fair value amount	10,247	10,149
Total financial assets	7,321,308	7,242,062
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	1,085,410	1,137,289
Securities sold under repurchase agreements	466,399	510,635
Other borrowed funds	15	2
Accrued interest payable	5,647	4,960
Long-term debt	27,535	27,622
Subordinated debentures held by subsidiary trusts	71,173	74,969
Derivative Liability	6,260	829
Deferred compensation plan liabilities at fair value amount	10,247	10,149
Total financial liabilities	1,672,686	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
Mortgages Held-for-sale, Fair Value Disclosure	0
Loans Receivable, Fair Value Disclosure	21,538	20,875
Derivative Asset	0	0
Deferred compensation asset at fair value amount	0	0
Total financial assets	21,538	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
Accrued interest payable	0	0
Long-term debt	0	0
Subordinated debentures held by subsidiary trusts	0	0
Derivative Liability	0	0
Deferred compensation plan liabilities at fair value amount	0	0
Total financial liabilities	$ 0	$ 0