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SECURITIES (Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Values of Investment Securities) (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
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Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | $ 2,352,916 | $ 1,893,427
Gross Unrealized Gains | 95,593 | 53,918
Gross Unrealized (Losses) | (5,169) | (1,900)
Total AFS securities, Estimated Fair Value | 2,443,340 | 1,945,445
U.S. government and agency securities |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 13,463 | 21,149
Gross Unrealized Gains | 465 | 209
Gross Unrealized (Losses) | (50) | (38)
Total AFS securities, Estimated Fair Value | 13,878 | 21,320
Obligations of states and political subdivisions |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 759,936 | 421,344
Gross Unrealized Gains | 39,033 | 25,776
Gross Unrealized (Losses) | (2,113) | (29)
Total AFS securities, Estimated Fair Value | 796,856 | 447,091
Corporate and other bonds |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 147,438 | 134,342
Gross Unrealized Gains | 2,217 | 1,991
Gross Unrealized (Losses) | (737) | (374)
Total AFS securities, Estimated Fair Value | 148,918 | 135,959
Commercial |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 371,065 | 416,904
Gross Unrealized Gains | 18,016 | 8,786
Gross Unrealized (Losses) | (116) | (643)
Total AFS securities, Estimated Fair Value | 388,965 | 425,047
Commercial | Agency |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 340,706 | 405,731
Gross Unrealized Gains | 17,945 | 8,786
Gross Unrealized (Losses) | (116) | (619)
Total AFS securities, Estimated Fair Value | 358,535 | 413,898
Commercial | Non-agency |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 30,359 | 11,173
Gross Unrealized Gains | 71 |
Gross Unrealized (Losses) |  | (24)
Total AFS securities, Estimated Fair Value | 30,430 | 11,149
Residential |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 1,059,393 | 896,609
Gross Unrealized Gains | 35,862 | 17,156
Gross Unrealized (Losses) | (2,153) | (816)
Total AFS securities, Estimated Fair Value | 1,093,102 | 912,949
Residential | Agency |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 960,141 | 852,300
Gross Unrealized Gains | 35,275 | 16,680
Gross Unrealized (Losses) | (1,410) | (816)
Total AFS securities, Estimated Fair Value | 994,006 | 868,164
Residential | Non-agency |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 99,252 | 44,309
Gross Unrealized Gains | 587 | 476
Gross Unrealized (Losses) | (743) |
Total AFS securities, Estimated Fair Value | 99,096 | 44,785
Other securities |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 1,621 | 3,079
Gross Unrealized Gains | 0 | 0
Gross Unrealized (Losses) | 0 | 0
Total AFS securities, Estimated Fair Value | $ 1,621 | $ 3,079

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