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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, 2021 | Dec. 31, 2020
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Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | $ 3,150,256 | $ 2,439,247
Gross Unrealized Gains | 70,084 | 103,512
Gross Unrealized (Losses) | (25,164) | (2,340)
Total AFS securities, Estimated Fair Value | 3,195,176 | 2,540,419
U.S. government and agency securities |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 74,214 | 13,009
Gross Unrealized Gains | 645 | 437
Gross Unrealized (Losses) | (41) | (52)
Total AFS securities, Estimated Fair Value | 74,818 | 13,394
Obligations of states and political subdivisions |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 971,987 | 786,466
Gross Unrealized Gains | 34,885 | 50,878
Gross Unrealized (Losses) | (7,991) | (18)
Total AFS securities, Estimated Fair Value | 998,881 | 837,326
Corporate and other bonds |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 145,151 | 148,747
Gross Unrealized Gains | 3,931 | 2,430
Gross Unrealized (Losses) | (54) | (99)
Total AFS securities, Estimated Fair Value | 149,028 | 151,078
Commercial |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 433,732 | 372,259
Gross Unrealized Gains | 9,463 | 16,444
Gross Unrealized (Losses) | (2,853) | (19)
Total AFS securities, Estimated Fair Value | 440,342 | 388,684
Commercial | Agency |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 338,327 | 321,015
Gross Unrealized Gains | 9,213 | 16,277
Gross Unrealized (Losses) | (2,557) | (2)
Total AFS securities, Estimated Fair Value | 344,983 | 337,290
Commercial | Non-agency |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 95,405 | 51,244
Gross Unrealized Gains | 250 | 167
Gross Unrealized (Losses) | (296) | (17)
Total AFS securities, Estimated Fair Value | 95,359 | 51,394
Residential |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 1,523,536 | 1,117,141
Gross Unrealized Gains | 21,160 | 33,323
Gross Unrealized (Losses) | (14,225) | (2,152)
Total AFS securities, Estimated Fair Value | 1,530,471 | 1,148,312
Residential | Agency |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 1,449,753 | 1,012,237
Gross Unrealized Gains | 20,630 | 31,816
Gross Unrealized (Losses) | (14,030) | (1,946)
Total AFS securities, Estimated Fair Value | 1,456,353 | 1,042,107
Residential | Non-agency |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 73,783 | 104,904
Gross Unrealized Gains | 530 | 1,507
Gross Unrealized (Losses) | (195) | (206)
Total AFS securities, Estimated Fair Value | 74,118 | 106,205
Other securities |  |
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract] |  |
Total AFS securities | 1,636 | 1,625
Gross Unrealized Gains | 0 | 0
Gross Unrealized (Losses) | 0 | 0
Total AFS securities, Estimated Fair Value | $ 1,636 | $ 1,625

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