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INVESTMENT SECURITIES, Investment Securities in a Continuous Unrealized Loss Position (Details) $ in Thousands | Mar. 31, 2018 USD ($) Position | Dec. 31, 2017 USD ($) Position
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Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | Position | 482 | 249
12 months or longer | Position | 59 | 56
Total | Position | 541 | 305
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 1,729,339 | $ 991,521
12 months or longer | 75,600 | 75,713
Total | 1,804,939 | 1,067,234
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 27,315 | 7,231
12 months or longer | 4,140 | 2,330
Total | $ 31,455 | $ 9,561
U.S. Treasury and Agency Securities [Member] |  |
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | Position | 69 | 44
12 months or longer | Position | 0 | 0
Total | Position | 69 | 44
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 1,316,113 | $ 699,709
12 months or longer | 0 | 0
Total | 1,316,113 | 699,709
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 18,843 | 4,838
12 months or longer | 0 | 0
Total | $ 18,843 | $ 4,838
Obligations of State and Political Subdivisions [Member] |  |
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | Position | 224 | 45
12 months or longer | Position | 0 | 0
Total | Position | 224 | 45
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 110,119 | $ 23,432
12 months or longer | 0 | 0
Total | 110,119 | 23,432
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 1,335 | 57
12 months or longer | 0 | 0
Total | $ 1,335 | $ 57
Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | Position | 148 | 120
12 months or longer | Position | 58 | 55
Total | Position | 206 | 175
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 225,378 | $ 185,716
12 months or longer | 75,599 | 75,712
Total | 300,977 | 261,428
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 5,310 | 1,433
12 months or longer | 4,140 | 2,330
Total | $ 9,450 | $ 3,763
Corporate Debt Securities [Member] |  |
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | Position | 1 | 1
12 months or longer | Position | 0 | 0
Total | Position | 1 | 1
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 2,579 | $ 2,623
12 months or longer | 0 | 0
Total | 2,579 | 2,623
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 54 | 25
12 months or longer | 0 | 0
Total | $ 54 | $ 25
Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | Position | 40 | 39
12 months or longer | Position | 1 | 1
Total | Position | 41 | 40
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 75,150 | $ 80,041
12 months or longer | 1 | 1
Total | 75,151 | 80,042
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 1,773 | 878
12 months or longer | 0 | 0
Total | $ 1,773 | $ 878

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