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INVESTMENT SECURITIES (Details) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Available-for-Sale Portfolio [Abstract] |  |
Amortized cost | $ 3,021,304 | $ 2,706,863
Gross unrealized gains | 55,827 | 47,463
Gross unrealized losses | 2,871 | 5,670
Fair value | 3,074,260 | 2,748,656
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 3,021,053 |
Fair value | 3,073,733 |
Other Securities [Abstract] |  |
Amortized cost | 50,958 | 35,736
Fair value | 50,958 | 35,736
U.S. Treasury and Agency Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 2,040,820 | 1,876,358
Gross unrealized gains | 33,703 | 28,522
Gross unrealized losses | 382 | 2,118
Fair value | 2,074,141 | 1,902,762
Obligations of State and Political Subdivisions [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 540,810 | 582,655
Gross unrealized gains | 17,175 | 13,389
Gross unrealized losses | 26 | 1,054
Fair value | 557,959 | 594,990
Government Agency Mortgage-Backed Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 342,838 | 232,657
Gross unrealized gains | 4,398 | 5,040
Gross unrealized losses | 2,314 | 2,467
Fair value | 344,922 | 235,230
Corporate Debt Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 2,663 | 5,716
Gross unrealized gains | 0 | 2
Gross unrealized losses | 1 | 31
Fair value | 2,662 | 5,687
Government Agency Collateralized Mortgage Obligations [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 93,922 | 9,225
Gross unrealized gains | 275 | 310
Gross unrealized losses | 148 | 0
Fair value | 94,049 | 9,535
Marketable Equity Securities [Member] |  |
Available For Sale Equity Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 251 | 252
Gross unrealized gains | 276 | 200
Gross unrealized losses | 0 | 0
Fair value | 527 | 452
Federal Home Loan Bank Common Stock [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 8,837 | 12,191
Fair value | 8,837 | 12,191
Federal Reserve Bank Common Stock [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 30,690 | 19,781
Fair value | 30,690 | 19,781
Certificates of Deposit [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 5,581 | 0
Fair value | 5,581 | 0
Other Equity Securities [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 5,850 | 3,764
Fair value | $ 5,850 | $ 3,764

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