Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Financial assets:
Investment securities held-to-maturity	$ 493,925	$ 513,273
Carrying Amount
Financial assets:
Cash and cash equivalents	807,952	782,023
Investment securities available-for-sale	1,657,591	1,611,698
Investment securities held-to-maturity	490,968	512,770
Accrued interest receivable	27,712	29,852
Mortgage servicing rights	19,454	18,457
Mortgages Held-for-sale, Fair Value Disclosure	23,237	61,794
Loans Receivable, Fair Value Disclosure	5,300,311	5,340,536
Derivative Asset	3,023	2,749
Deferred compensation asset at fair value amount	11,108	10,627
Total financial assets	8,341,356	8,370,506
Financial liabilities:
Total deposits, excluding time deposits	6,299,343	6,324,512
Time deposits	1,000,836	1,051,598
Securities sold under repurchase agreements	587,570	537,556
Other borrowed funds	1	6
Accrued interest payable	4,831	5,421
Long-term debt	27,994	27,970
Subordinated debentures held by subsidiary trusts	82,477	82,477
Derivative Liability	1,688	1,314
Deferred compensation plan liabilities at fair value amount	11,108	10,627
Total financial liabilities	8,015,848	8,041,481
Estimated Fair Value
Financial assets:
Cash and cash equivalents	807,952	782,023
Investment securities available-for-sale	1,657,591	1,611,698
Investment securities held-to-maturity	493,925	513,273
Accrued interest receivable	27,712	29,852
Mortgage servicing rights	36,151	35,656
Mortgages Held-for-sale, Fair Value Disclosure	23,237	61,794
Loans Receivable, Fair Value Disclosure	5,148,977	5,248,127
Derivative Asset	3,023	2,749
Deferred compensation asset at fair value amount	11,108	10,627
Total financial assets	8,209,676	8,295,799
Financial liabilities:
Total deposits, excluding time deposits	6,299,343	6,324,512
Time deposits	992,746	1,044,670
Securities sold under repurchase agreements	587,570	537,556
Other borrowed funds	1	6
Accrued interest payable	4,831	5,421
Long-term debt	27,520	27,477
Subordinated debentures held by subsidiary trusts	85,442	73,554
Derivative Liability	1,688	1,314
Deferred compensation plan liabilities at fair value amount	11,108	10,627
Total financial liabilities	8,010,249	8,025,137
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	807,952	782,023
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	0
Loans Receivable, Fair Value Disclosure	0	0
Derivative Asset	0	0
Deferred compensation asset at fair value amount	0	0
Total financial assets	807,952	782,023
Financial liabilities:
Total deposits, excluding time deposits	6,299,343	6,324,512
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	6,299,343	6,324,512
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment securities available-for-sale	1,657,591	1,611,698
Investment securities held-to-maturity	493,925	513,273
Accrued interest receivable	27,712	29,852
Mortgage servicing rights	36,151	35,656
Mortgages Held-for-sale, Fair Value Disclosure	23,237	61,794
Loans Receivable, Fair Value Disclosure	5,111,493	5,208,783
Derivative Asset	3,023	2,749
Deferred compensation asset at fair value amount	11,108	10,627
Total financial assets	7,364,240	7,474,432
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	992,746	1,044,670
Securities sold under repurchase agreements	587,570	537,556
Other borrowed funds	1	6
Accrued interest payable	4,831	5,421
Long-term debt	27,520	27,477
Subordinated debentures held by subsidiary trusts	85,442	73,554
Derivative Liability	1,688	1,314
Deferred compensation plan liabilities at fair value amount	11,108	10,627
Total financial liabilities	1,710,906	1,700,625
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	37,484	39,344
Derivative Asset	0	0
Deferred compensation asset at fair value amount	0	0
Total financial assets	37,484	39,344
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