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INVESTMENT SECURITIES (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Available-for-Sale Portfolio [Abstract] |  |
Amortized cost | $ 3,007,148 | $ 2,706,863
Gross unrealized gains | 33,501 | 47,463
Gross unrealized losses | 9,561 | 5,670
Fair value | 3,031,088 | 2,748,656
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 3,006,897 |
Estimated fair value | 3,030,562 |
Other Securities [Abstract] |  |
Amortized cost | 50,291 | 35,736
Estimated fair value | 50,291 | 35,736
U.S. Treasury and Agency Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 2,043,023 | 1,876,358
Gross unrealized gains | 15,886 | 28,522
Gross unrealized losses | 4,838 | 2,118
Estimated fair value | 2,054,071 | 1,902,762
Obligations of State and Political Subdivisions [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 514,949 | 582,655
Gross unrealized gains | 14,064 | 13,389
Gross unrealized losses | 57 | 1,054
Estimated fair value | 528,956 | 594,990
Government Agency Mortgage-Backed Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 358,180 | 232,657
Gross unrealized gains | 3,121 | 5,040
Gross unrealized losses | 3,763 | 2,467
Estimated fair value | 357,538 | 235,230
Corporate Debt Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 2,648 | 5,716
Gross unrealized gains | 0 | 2
Gross unrealized losses | 25 | 31
Estimated fair value | 2,623 | 5,687
Government Agency Collateralized Mortgage Obligations [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 88,097 | 9,225
Gross unrealized gains | 155 | 310
Gross unrealized losses | 878 | 0
Estimated fair value | 87,374 | 9,535
Marketable Equity Security [Member] |  |
Available For Sale Equity Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 251 | 252
Gross unrealized gains | 275 | 200
Gross unrealized losses | 0 | 0
Estimated fair value | 526 | 452
Federal Home Loan Bank Common Stock [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 9,896 | 12,191
Estimated fair value | 9,896 | 12,191
Federal Reserve Bank Common Stock [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 30,690 | 19,781
Estimated fair value | 30,690 | 19,781
Certificates of Deposit [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 3,865 | 0
Estimated fair value | 3,865 | 0
Other Equity Securities [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 5,840 | 3,764
Estimated fair value | $ 5,840 | $ 3,764

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