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INVESTMENT SECURITIES (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Available-for-Sale Portfolio [Abstract] |  |
Amortized cost | $ 3,005,165 | $ 3,007,148
Gross unrealized gains | 13,580 | 33,501
Gross unrealized losses | 31,455 | 9,561
Fair value | 2,987,290 | 3,031,088
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 3,004,915 |
Fair value | 2,986,769 |
Other Securities [Abstract] |  |
Amortized cost | 45,352 | 50,291
Fair value | 45,352 | 50,291
U.S. Treasury and Agency Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 2,044,795 | 2,043,023
Gross unrealized gains | 1,962 | 15,886
Gross unrealized losses | 18,843 | 4,838
Fair value | 2,027,914 | 2,054,071
Obligations of State and Political Subdivisions [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 502,517 | 514,949
Gross unrealized gains | 9,126 | 14,064
Gross unrealized losses | 1,335 | 57
Fair value | 510,308 | 528,956
Government Agency Mortgage-Backed Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 371,857 | 358,180
Gross unrealized gains | 2,132 | 3,121
Gross unrealized losses | 9,450 | 3,763
Fair value | 364,539 | 357,538
Corporate Debt Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 2,633 | 2,648
Gross unrealized gains | 0 | 0
Gross unrealized losses | 54 | 25
Fair value | 2,579 | 2,623
Government Agency Collateralized Mortgage Obligations [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 83,113 | 88,097
Gross unrealized gains | 89 | 155
Gross unrealized losses | 1,773 | 878
Fair value | 81,429 | 87,374
Marketable Equity Securities [Member] |  |
Available For Sale Equity Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 250 | 251
Gross unrealized gains | 271 | 275
Gross unrealized losses | 0 | 0
Fair value | 521 | 526
Federal Home Loan Bank Common Stock [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 8,801 | 9,896
Fair value | 8,801 | 9,896
Federal Reserve Bank Common Stock [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 30,690 | 30,690
Fair value | 30,690 | 30,690
Certificates of Deposit [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 0 | 3,865
Fair value | 0 | 3,865
Other Equity Securities [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 5,861 | 5,840
Fair value | $ 5,861 | $ 5,840

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