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INVESTMENT SECURITIES (Details) - USD ($) $ in Thousands | Jun. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Available-for-Sale Portfolio [Abstract] |  |
Amortized cost | $ 3,030,878 | $ 2,706,863
Gross unrealized gains | 58,612 | 47,463
Gross unrealized losses | 3,166 | 5,670
Fair value | 3,086,324 | 2,748,656
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 3,030,627 |
Fair value | 3,085,827 |
Other Securities [Abstract] |  |
Amortized cost | 58,689 | 35,736
Fair value | 58,689 | 35,736
U.S. Treasury and Agency Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 2,039,313 | 1,876,358
Gross unrealized gains | 36,666 | 28,522
Gross unrealized losses | 181 | 2,118
Fair value | 2,075,798 | 1,902,762
Obligations of State and Political Subdivisions [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 556,459 | 582,655
Gross unrealized gains | 17,080 | 13,389
Gross unrealized losses | 67 | 1,054
Fair value | 573,472 | 594,990
Government Agency Mortgage-Backed Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 331,925 | 232,657
Gross unrealized gains | 4,326 | 5,040
Gross unrealized losses | 2,804 | 2,467
Fair value | 333,447 | 235,230
Corporate Debt Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 2,678 | 5,716
Gross unrealized gains | 0 | 2
Gross unrealized losses | 7 | 31
Fair value | 2,671 | 5,687
Government Agency Collateralized Mortgage Obligations [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 100,252 | 9,225
Gross unrealized gains | 294 | 310
Gross unrealized losses | 107 | 0
Fair value | 100,439 | 9,535
Marketable Equity Securities [Member] |  |
Available For Sale Equity Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 251 | 252
Gross unrealized gains | 246 | 200
Gross unrealized losses | 0 | 0
Fair value | 497 | 452
Federal Home Loan Bank Common Stock [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 18,439 | 12,191
Fair value | 18,439 | 12,191
Federal Reserve Bank Common Stock [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 19,806 | 19,781
Fair value | 19,806 | 19,781
Certificates of Deposit [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 14,450 | 0
Fair value | 14,450 | 0
Other Equity Securities [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 5,994 | 3,764
Fair value | $ 5,994 | $ 3,764

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