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INVESTMENT SECURITIES, Available-for-Sale Portfolio (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Available-for-Sale Portfolio [Abstract] |  |
Amortized cost | $ 2,972,889 | $ 3,007,148
Gross unrealized gains | 10,078 | 33,501
Gross unrealized losses | 43,023 | 9,561
Fair value | 2,939,944 | 3,031,088
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |
Amortized cost | 2,972,889 |
Fair value | 2,939,944 |
Marketable Equity Securities [Abstract] |  |
Amortized cost |  | 251
Gross unrealized gains |  | 275
Gross unrealized losses |  | 0
Fair value |  | 526
U.S. Treasury and Agency Securities [Member] |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |
Amortized cost | 2,042,116 | 2,043,023
Gross unrealized gains | 477 | 15,886
Gross unrealized losses | 28,631 | 4,838
Fair value | 2,013,962 | 2,054,071
Obligations of State and Political Subdivisions [Member] |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |
Amortized cost | 485,099 | 514,949
Gross unrealized gains | 7,825 | 14,064
Gross unrealized losses | 1,440 | 57
Fair value | 491,484 | 528,956
Government Agency Mortgage-Backed Securities [Member] |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |
Amortized cost | 364,919 | 358,180
Gross unrealized gains | 1,724 | 3,121
Gross unrealized losses | 10,746 | 3,763
Fair value | 355,897 | 357,538
Corporate Debt Securities [Member] |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |
Amortized cost | 2,618 | 2,648
Gross unrealized gains | 0 | 0
Gross unrealized losses | 55 | 25
Fair value | 2,563 | 2,623
Government Agency Collateralized Mortgage Obligations [Member] |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |
Amortized cost | 78,137 | 88,097
Gross unrealized gains | 52 | 155
Gross unrealized losses | 2,151 | 878
Fair value | $ 76,038 | $ 87,374

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