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FAIR VALUE, Financial Assets and Liabilities Accounted for at Fair Value On a Recurring Basis (Details) - USD ($) $ in Thousands | Sep. 30, 2015 | Dec. 31, 2014
--- | --- | ---
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | $ 2,867,439 | $ 2,472,925
Recurring [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,867,439 | 2,472,925
Mortgages loans held for sale | 1,014 | 1,042
Commitments to originate real estate loans for sale | 276 | 185
Forward sales commitments | (94) | (43)
Total | 2,868,635 | 2,474,109
Recurring [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 1,933,270 | 1,517,733
Recurring [Member] | Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 686,627 | 671,903
Recurring [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 215,936 | 237,728
Recurring [Member] | Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 16,782 | 27,091
Recurring [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 14,418 | 18,025
Recurring [Member] | Marketable Equity Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 406 | 445
Recurring [Member] | Level 1 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 1,933,676 | 1,497,112
Mortgages loans held for sale | 0 | 0
Commitments to originate real estate loans for sale | 0 | 0
Forward sales commitments | 0 | 0
Total | 1,933,676 | 1,497,112
Recurring [Member] | Level 1 [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 1,933,270 | 1,496,667
Recurring [Member] | Level 1 [Member] | Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 1 [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 1 [Member] | Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 1 [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 1 [Member] | Marketable Equity Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 406 | 445
Recurring [Member] | Level 2 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 933,763 | 975,813
Mortgages loans held for sale | 1,014 | 1,042
Commitments to originate real estate loans for sale | 0 | 0
Forward sales commitments | (94) | (43)
Total | 934,683 | 976,812
Mortgage loans held for sale, at principal value | 1,000 |
Recurring [Member] | Level 2 [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 21,066
Recurring [Member] | Level 2 [Member] | Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 686,627 | 671,903
Recurring [Member] | Level 2 [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 215,936 | 237,728
Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 16,782 | 27,091
Recurring [Member] | Level 2 [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 14,418 | 18,025
Recurring [Member] | Level 2 [Member] | Marketable Equity Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 3 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Mortgages loans held for sale | 0 | 0
Commitments to originate real estate loans for sale | 276 | 185
Forward sales commitments | 0 | 0
Total | 276 | 185
Recurring [Member] | Level 3 [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 3 [Member] | Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 3 [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 3 [Member] | Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 3 [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 3 [Member] | Marketable Equity Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | $ 0 | $ 0

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