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INVESTMENT SECURITIES (Details) - USD ($) $ in Thousands | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | ---
Available-for-Sale Portfolio [Abstract] |  |
Amortized cost | $ 2,706,863 | $ 2,742,278
Gross unrealized gains | 47,463 | 69,254
Gross unrealized losses | 5,670 | 3,419
Fair value | 2,748,656 | 2,808,113
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 2,706,611 |
Estimated fair value | 2,748,204 |
Other Securities [Abstract] |  |
Amortized cost | 35,736 | 39,827
Estimated fair value | 35,736 | 39,827
U.S. Treasury and Agency Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 1,876,358 | 1,866,819
Gross unrealized gains | 28,522 | 35,186
Gross unrealized losses | 2,118 | 2,027
Estimated fair value | 1,902,762 | 1,899,978
Obligations of State and Political Subdivisions [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 582,655 | 640,455
Gross unrealized gains | 13,389 | 26,487
Gross unrealized losses | 1,054 | 59
Estimated fair value | 594,990 | 666,883
Government Agency Mortgage-Backed Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 232,657 | 205,220
Gross unrealized gains | 5,040 | 6,906
Gross unrealized losses | 2,467 | 1,261
Estimated fair value | 235,230 | 210,865
Corporate Debt Securities [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 5,716 | 16,672
Gross unrealized gains | 2 | 66
Gross unrealized losses | 31 | 58
Estimated fair value | 5,687 | 16,680
Government Agency Collateralized Mortgage Obligations [Member] |  |
Available For Sale Debt Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 9,225 | 12,862
Gross unrealized gains | 310 | 446
Gross unrealized losses | 0 | 0
Estimated fair value | 9,535 | 13,308
Marketable Equity Security [Member] |  |
Available For Sale Equity Securities Amortized Cost Basis [Abstract] |  |
Amortized cost | 252 | 250
Gross unrealized gains | 200 | 163
Gross unrealized losses | 0 | 14
Estimated fair value | 452 | 399
Federal Home Loan Bank Common Stock [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 12,191 | 19,317
Estimated fair value | 12,191 | 19,317
Federal Reserve Bank Common Stock [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 19,781 | 16,050
Estimated fair value | 19,781 | 16,050
Other Equity Securities [Member] |  |
Other Securities [Abstract] |  |
Amortized cost | 3,764 | 4,460
Estimated fair value | $ 3,764 | $ 4,460

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