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INVESTMENT SECURITIES - Amortized cost and estimated fair value of investment securities (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022
Available-for-Sale Portfolio: |  |
Amortized Cost | $ 3,301,607 | $ 4,675,474
Gross Unrealized Gains | 1,545 | 565
Gross Unrealized Losses | 383,160 | 524,188
Fair Value | 2,919,992 | 4,151,851
Held-to-Maturity Portfolio: |  |
Amortized Cost | 1,172,174 | 1,079,695
Gross Unrealized Gains | 688 | 0
Gross Unrealized Losses | 51,046 | 44,900
Fair Value | 1,121,816 | 1,034,795
Equity securities, at fair value |  |
Amortized Cost | 251 | 251
Gross Unrealized Gains | 121 | 168
Gross Unrealized Losses | 0 | 0
Fair Value | 372 | 419
Equity and other Securities: |  |
Amortized cost | 72,275 | 82,424
Gross Unrealized Gains | 871 | 918
Gross Unrealized Losses | 0 | 0
Equity and other securities | 73,146 | 83,342
U.S. Treasury and agency securities |  |
Available-for-Sale Portfolio: |  |
Amortized Cost | 2,381,168 | 3,660,546
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 300,385 | 417,009
Fair Value | 2,080,783 | 3,243,537
Held-to-Maturity Portfolio: |  |
Amortized Cost | 1,109,101 | 1,079,695
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 50,866 | 44,900
Fair Value | 1,058,235 | 1,034,795
Obligations of state and political subdivisions |  |
Available-for-Sale Portfolio: |  |
Amortized Cost | 502,879 | 549,118
Gross Unrealized Gains | 1,469 | 506
Gross Unrealized Losses | 29,985 | 45,327
Fair Value | 474,363 | 504,297
Government agency mortgage-backed securities |  |
Available-for-Sale Portfolio: |  |
Amortized Cost | 400,062 | 444,689
Gross Unrealized Gains | 76 | 58
Gross Unrealized Losses | 51,612 | 60,114
Fair Value | 348,526 | 384,633
Held-to-Maturity Portfolio: |  |
Amortized Cost | 63,073 | 0
Gross Unrealized Gains | 688 | 0
Gross Unrealized Losses | 180 | 0
Fair Value | 63,581 | 0
Corporate debt securities |  |
Available-for-Sale Portfolio: |  |
Amortized Cost | 8,000 | 8,000
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 606 | 886
Fair Value | 7,394 | 7,114
Government agency collateralized mortgage obligations |  |
Available-for-Sale Portfolio: |  |
Amortized Cost | 9,498 | 13,121
Gross Unrealized Gains | 0 | 1
Gross Unrealized Losses | 572 | 852
Fair Value | 8,926 | 12,270
Held-to-Maturity Portfolio: |  |
Amortized Cost | 0 |
Fair Value | 0 |
Federal Home Loan Bank Common Stock |  |
Other equity securities, at adjusted cost |  |
Amortized Cost | 32,526 | 47,497
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 32,526 | 47,497
Federal Reserve Bank Common Stock |  |
Other equity securities, at adjusted cost |  |
Amortized Cost | 33,568 | 31,144
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 33,568 | 31,144
Other Equity Securities |  |
Other equity securities, at adjusted cost |  |
Amortized Cost | 5,930 | 3,532
Gross Unrealized Gains | 750 | 750
Gross Unrealized Losses | 0 | 0
Fair Value | $ 6,680 | $ 4,282

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