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INVESTMENT SECURITIES (Details) - USD ($) $ in Thousands | Mar. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Available-for-Sale Portfolio [Abstract] |  |
Amortized cost | $ 2,695,204 | $ 2,706,863
Gross unrealized gains | 49,983 | 47,463
Gross unrealized losses | 4,297 | 5,670
Fair value | 2,740,890 | 2,748,656
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 2,694,953 |
Fair value | 2,740,436 |
Other Securities [Abstract] |  |
Amortized cost | 47,828 | 35,736
Fair value | 47,828 | 35,736
U.S. Treasury and Agency Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 1,878,982 | 1,876,358
Gross unrealized gains | 30,358 | 28,522
Gross unrealized losses | 1,106 | 2,118
Fair value | 1,908,234 | 1,902,762
Obligations of State and Political Subdivisions [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 564,501 | 582,655
Gross unrealized gains | 14,603 | 13,389
Gross unrealized losses | 446 | 1,054
Fair value | 578,658 | 594,990
Government Agency Mortgage-Backed Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 237,257 | 232,657
Gross unrealized gains | 4,526 | 5,040
Gross unrealized losses | 2,729 | 2,467
Fair value | 239,054 | 235,230
Corporate Debt Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 5,697 | 5,716
Gross unrealized gains | 0 | 2
Gross unrealized losses | 16 | 31
Fair value | 5,681 | 5,687
Government Agency Collateralized Mortgage Obligations [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 8,516 | 9,225
Gross unrealized gains | 293 | 310
Gross unrealized losses | 0 | 0
Fair value | 8,809 | 9,535
Marketable Equity Securities [Member] |  |
Available For Sale Equity Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 251 | 252
Gross unrealized gains | 203 | 200
Gross unrealized losses | 0 | 0
Fair value | 454 | 452
Federal Home Loan Bank Common Stock [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 5,590 | 12,191
Fair value | 5,590 | 12,191
Federal Reserve Bank Common Stock [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 19,781 | 19,781
Fair value | 19,781 | 19,781
Certificates of Deposit [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 18,758 | 0
Fair value | 18,758 | 0
Other Equity Securities [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 3,699 | 3,764
Fair value | $ 3,699 | $ 3,764

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