Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Financial assets:
Investment securities held-to-maturity	$ 588,032	$ 205,926
Carrying Amount
Financial assets:
Cash and cash equivalents	819,963	534,827
Investment securities available-for-sale	1,586,503	1,947,706
Investment securities held-to-maturity	583,271	203,837
Accrued interest receivable	30,282	26,450
Mortgage servicing rights	13,894	13,546
Net loans	4,780,151	4,259,514
Total financial assets	7,814,064	6,985,880
Financial liabilities:
Total deposits, excluding time deposits	5,705,490	4,943,033
Time deposits	1,254,009	1,190,717
Securities sold under repurchase agreements	432,478	457,437
Other borrowed funds	1,934	3
Accrued interest payable	5,569	4,963
Long-term debt	36,882	36,917
Subordinated debentures held by subsidiary trusts	102,916	82,477
Total financial liabilities	7,539,278	6,715,547
Estimated Fair Value
Financial assets:
Cash and cash equivalents	819,963	534,827
Investment securities available-for-sale	1,586,503	1,947,706
Investment securities held-to-maturity	588,032	205,926
Accrued interest receivable	30,282	26,450
Mortgage servicing rights	27,292	25,698
Net loans	4,784,957	4,246,539
Total financial assets	7,837,029	6,987,146
Financial liabilities:
Total deposits, excluding time deposits	5,705,490	4,943,033
Time deposits	1,259,444	1,196,250
Securities sold under repurchase agreements	432,478	457,437
Other borrowed funds	1,934	3
Accrued interest payable	5,569	4,963
Long-term debt	36,524	34,508
Subordinated debentures held by subsidiary trusts	98,177	72,045
Total financial liabilities	7,539,616	6,708,239
Quoted Prices in Active Markets for Identical Assets (Level 1) | 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	0	0
Total financial assets	0	0
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
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	819,963	534,827
Investment securities available-for-sale	1,586,503	1,947,706
Investment securities held-to-maturity	588,032	205,926
Accrued interest receivable	30,282	26,450
Mortgage servicing rights	27,292	25,698
Net loans	4,742,961	4,189,237
Total financial assets	7,795,033	6,929,844
Financial liabilities:
Total deposits, excluding time deposits	5,705,490	4,943,033
Time deposits	1,259,444	1,196,250
Securities sold under repurchase agreements	432,478	457,437
Other borrowed funds	1,934	3
Accrued interest payable	5,569	4,963
Long-term debt	36,524	34,508
Subordinated debentures held by subsidiary trusts	98,177	72,045
Total financial liabilities	7,539,616	6,708,239
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	41,996	57,302
Total financial assets	41,996	57,302
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