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SECURITIES (Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Values of Investment Securities) (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | $ 2,439,247 | $ 1,893,427
Gross Unrealized Gains | 103,512 | 53,918
Gross Unrealized (Losses) | (2,340) | (1,900)
Total AFS securities, Estimated Fair Value | 2,540,419 | 1,945,445
U.S. government and agency securities |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 13,009 | 21,149
Gross Unrealized Gains | 437 | 209
Gross Unrealized (Losses) | (52) | (38)
Total AFS securities, Estimated Fair Value | 13,394 | 21,320
Obligations of states and political subdivisions |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 786,466 | 421,344
Gross Unrealized Gains | 50,878 | 25,776
Gross Unrealized (Losses) | (18) | (29)
Total AFS securities, Estimated Fair Value | 837,326 | 447,091
Corporate and other bonds |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 148,747 | 134,342
Gross Unrealized Gains | 2,430 | 1,991
Gross Unrealized (Losses) | (99) | (374)
Total AFS securities, Estimated Fair Value | 151,078 | 135,959
Commercial |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 372,259 | 416,904
Gross Unrealized Gains | 16,444 | 8,786
Gross Unrealized (Losses) | (19) | (643)
Total AFS securities, Estimated Fair Value | 388,684 | 425,047
Commercial | Agency |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 321,015 | 405,731
Gross Unrealized Gains | 16,277 | 8,786
Gross Unrealized (Losses) | (2) | (619)
Total AFS securities, Estimated Fair Value | 337,290 | 413,898
Commercial | Non-agency |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 51,244 | 11,173
Gross Unrealized Gains | 167 | 0
Gross Unrealized (Losses) | (17) | (24)
Total AFS securities, Estimated Fair Value | 51,394 | 11,149
Residential |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 1,117,141 | 896,609
Gross Unrealized Gains | 33,323 | 17,156
Gross Unrealized (Losses) | (2,152) | (816)
Total AFS securities, Estimated Fair Value | 1,148,312 | 912,949
Residential | Agency |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 1,012,237 | 852,300
Gross Unrealized Gains | 31,816 | 16,680
Gross Unrealized (Losses) | (1,946) | (816)
Total AFS securities, Estimated Fair Value | 1,042,107 | 868,164
Residential | Non-agency |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 104,904 | 44,309
Gross Unrealized Gains | 1,507 | 476
Gross Unrealized (Losses) | (206) | 0
Total AFS securities, Estimated Fair Value | 106,205 | 44,785
Other securities |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 1,625 | 3,079
Gross Unrealized Gains | 0 | 0
Gross Unrealized (Losses) | 0 | 0
Total AFS securities, Estimated Fair Value | $ 1,625 | $ 3,079

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