Fair Value Measurements - Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Financial assets:
Held-to-Maturity Estimated Fair Value	$ 584,533	$ 205,926
Carrying Amount
Financial assets:
Cash and cash equivalents	798,670	534,827
Investment securities available-for-sale	1,711,924	1,947,706
Held-to-Maturity Estimated Fair Value	575,186	203,837
Accrued interest receivable	27,063	26,450
Mortgage servicing rights	14,038	13,546
Net loans	4,823,243	4,259,514
Total financial assets	7,950,124	6,985,880
Financial liabilities:
Total deposits, excluding time deposits	5,767,992	4,943,033
Time deposits	1,238,220	1,190,717
Securities sold under repurchase agreements	502,250	457,437
Other borrowed funds	9	3
Accrued interest payable	5,833	4,963
Long-term debt	38,067	36,917
Subordinated debentures held by subsidiary trusts	82,477	82,477
Total financial liabilities	7,634,848	6,715,547
Estimated Fair Value
Financial assets:
Cash and cash equivalents	798,670	534,827
Investment securities available-for-sale	1,711,924	1,947,706
Held-to-Maturity Estimated Fair Value	584,533	205,926
Accrued interest receivable	27,063	26,450
Mortgage servicing rights	21,434	25,698
Net loans	4,800,725	4,246,539
Total financial assets	7,944,349	6,987,146
Financial liabilities:
Total deposits, excluding time deposits	5,767,992	4,943,033
Time deposits	1,244,324	1,196,250
Securities sold under repurchase agreements	502,250	457,437
Other borrowed funds	9	3
Accrued interest payable	5,833	4,963
Long-term debt	37,781	34,508
Subordinated debentures held by subsidiary trusts	75,734	72,045
Total financial liabilities	7,633,923	6,708,239
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	798,670	534,827
Investment securities available-for-sale	0	0
Held-to-Maturity Estimated Fair Value	0	0
Accrued interest receivable	0	0
Mortgage servicing rights	0	0
Net loans	0	0
Total financial assets	798,670	534,827
Financial liabilities:
Total deposits, excluding time deposits	5,767,992	4,943,033
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	5,767,992	4,943,033
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment securities available-for-sale	1,711,924	1,947,706
Held-to-Maturity Estimated Fair Value	584,533	205,926
Accrued interest receivable	27,063	26,450
Mortgage servicing rights	21,434	25,698
Net loans	4,770,231	4,189,237
Total financial assets	7,115,185	6,395,017
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	1,244,324	1,196,250
Securities sold under repurchase agreements	502,250	457,437
Other borrowed funds	9	3
Accrued interest payable	5,833	4,963
Long-term debt	37,781	34,508
Subordinated debentures held by subsidiary trusts	75,734	72,045
Total financial liabilities	1,865,931	1,765,206
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment securities available-for-sale	0	0
Held-to-Maturity Estimated Fair Value	0	0
Accrued interest receivable	0	0
Mortgage servicing rights	0	0
Net loans	30,494	57,302
Total financial assets	30,494	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