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FAIR VALUE, Financial Assets and Liabilities Accounted for at Fair Value On a Recurring Basis (Details) - USD ($) $ in Thousands | Jun. 30, 2015 | Dec. 31, 2014
--- | --- | ---
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | $ 2,817,861 | $ 2,472,925
Recurring [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,817,861 | 2,472,925
Mortgages loans held for sale | 1,031 | 1,042
Commitments to originate real estate loans for sale | 195 | 185
Forward sales commitments | (81) | (43)
Total | 2,819,006 | 2,474,109
Recurring [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 1,893,445 | 1,517,733
Recurring [Member] | Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 662,764 | 671,903
Recurring [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 218,755 | 237,728
Recurring [Member] | Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 26,817 | 27,091
Recurring [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 15,655 | 18,025
Recurring [Member] | Marketable Equity Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 425 | 445
Recurring [Member] | Level 1 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 1,893,870 | 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 | 1,893,870 | 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 | 1,893,445 | 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 | 425 | 445
Recurring [Member] | Level 2 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 923,991 | 975,813
Mortgages loans held for sale | 1,031 | 1,042
Commitments to originate real estate loans for sale | 0 | 0
Forward sales commitments | (81) | (43)
Total | 924,941 | 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 | 662,764 | 671,903
Recurring [Member] | Level 2 [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 218,755 | 237,728
Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 26,817 | 27,091
Recurring [Member] | Level 2 [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 15,655 | 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 | 195 | 185
Forward sales commitments | 0 | 0
Total | 195 | 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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