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SECURITIES (Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Values of Investment Securities) (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
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Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | $ 2,640,063 | $ 2,439,247
Gross Unrealized Gains | 78,131 | 103,512
Gross Unrealized (Losses) | (21,151) | (2,340)
Total AFS securities, Estimated Fair Value | 2,697,043 | 2,540,419
U.S. government and agency securities |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 11,470 | 13,009
Gross Unrealized Gains | 254 | 437
Gross Unrealized (Losses) | (47) | (52)
Total AFS securities, Estimated Fair Value | 11,677 | 13,394
Obligations of states and political subdivisions |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 868,297 | 786,466
Gross Unrealized Gains | 35,757 | 50,878
Gross Unrealized (Losses) | (4,956) | (18)
Total AFS securities, Estimated Fair Value | 899,098 | 837,326
Corporate and other bonds |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 145,403 | 148,747
Gross Unrealized Gains | 2,448 | 2,430
Gross Unrealized (Losses) | (243) | (99)
Total AFS securities, Estimated Fair Value | 147,608 | 151,078
Commercial |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 375,218 | 372,259
Gross Unrealized Gains | 11,339 | 16,444
Gross Unrealized (Losses) | (1,725) | (19)
Total AFS securities, Estimated Fair Value | 384,832 | 388,684
Commercial | Agency |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 316,527 | 321,015
Gross Unrealized Gains | 11,260 | 16,277
Gross Unrealized (Losses) | (1,615) | (2)
Total AFS securities, Estimated Fair Value | 326,172 | 337,290
Commercial | Non-agency |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 58,691 | 51,244
Gross Unrealized Gains | 79 | 167
Gross Unrealized (Losses) | (110) | (17)
Total AFS securities, Estimated Fair Value | 58,660 | 51,394
Residential |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 1,238,050 | 1,117,141
Gross Unrealized Gains | 28,333 | 33,323
Gross Unrealized (Losses) | (14,180) | (2,152)
Total AFS securities, Estimated Fair Value | 1,252,203 | 1,148,312
Residential | Agency |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 1,138,014 | 1,012,237
Gross Unrealized Gains | 26,812 | 31,816
Gross Unrealized (Losses) | (13,668) | (1,946)
Total AFS securities, Estimated Fair Value | 1,151,158 | 1,042,107
Residential | Non-agency |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 100,036 | 104,904
Gross Unrealized Gains | 1,521 | 1,507
Gross Unrealized (Losses) | (512) | (206)
Total AFS securities, Estimated Fair Value | 101,045 | 106,205
Other securities |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 1,625 | 1,625
Gross Unrealized Gains | 0 | 0
Gross Unrealized (Losses) | 0 | 0
Total AFS securities, Estimated Fair Value | $ 1,625 | $ 1,625

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