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INVESTMENT SECURITIES (Details) - USD ($) $ in Thousands | Sep. 30, 2015 | Dec. 31, 2014
--- | --- | ---
Available-for-Sale Portfolio [Abstract] |  |
Amortized Cost | $ 2,766,581 | $ 2,397,886
Gross Unrealized Gains | 102,396 | 77,293
Gross Unrealized Losses | 1,538 | 2,254
Fair Value | 2,867,439 | 2,472,925
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized Cost | 2,766,331 |
Fair Value | 2,867,033 |
Other Securities [Abstract] |  |
Amortized Cost | 49,824 | 40,049
Fair Value | 49,824 | 40,049
U.S. Treasury and Agency Obligations [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized Cost | 1,864,199 | 1,479,134
Gross Unrealized Gains | 69,071 | 39,509
Gross Unrealized Losses | 0 | 910
Fair Value | 1,933,270 | 1,517,733
Obligations of State and Political Subdivisions [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized Cost | 662,774 | 645,398
Gross Unrealized Gains | 24,374 | 26,749
Gross Unrealized Losses | 521 | 244
Fair Value | 686,627 | 671,903
Government Agency Mortgage-Backed Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized Cost | 208,817 | 228,971
Gross Unrealized Gains | 8,087 | 9,782
Gross Unrealized Losses | 968 | 1,025
Fair Value | 215,936 | 237,728
Corporate Debt Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized Cost | 16,705 | 26,803
Gross Unrealized Gains | 115 | 363
Gross Unrealized Losses | 38 | 75
Fair Value | 16,782 | 27,091
Government Agency Collateralized Mortgage Obligations [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized Cost | 13,836 | 17,330
Gross Unrealized Gains | 582 | 695
Gross Unrealized Losses | 0 | 0
Fair Value | 14,418 | 18,025
Marketable Equity Securities [Member] |  |
Available For Sale Equity Securities Amortized Cost Basis [Abstract] |  |
Amortized Cost | 250 | 250
Gross Unrealized Gains | 167 | 195
Gross Unrealized Losses | 11 | 0
Fair Value | 406 | 445
Federal Home Loan Bank Common Stock [Member] |  |
Other Securities [Abstract] |  |
Amortized Cost | 29,500 | 19,553
Fair Value | 29,500 | 19,553
Federal Reserve Bank Common Stock [Member] |  |
Other Securities [Abstract] |  |
Amortized Cost | 16,050 | 16,050
Fair Value | 16,050 | 16,050
Other Equity Securities [Member] |  |
Other Securities [Abstract] |  |
Amortized Cost | 4,274 | 4,446
Fair Value | $ 4,274 | $ 4,446

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