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INVESTMENT SECURITIES - Amortized cost and estimated fair value of investment securities (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2024 | Dec. 31, 2023
Available-for-Sale Portfolio: |  |
Amortized Cost | $ 3,287,497 | $ 3,301,607
Gross Unrealized Gains | 676 | 1,545
Gross Unrealized Losses | 424,309 | 383,160
Fair Value | 2,863,864 | 2,919,992
Held-to-Maturity Portfolio: |  |
Amortized Cost | 1,217,472 | 1,172,174
Gross Unrealized Gains | 477 | 688
Gross Unrealized Losses | 86,334 | 51,046
Fair Value | 1,131,615 | 1,121,816
Equity securities, at fair value |  |
Amortized Cost | 252 | 251
Gross Unrealized Gains | 136 | 121
Gross Unrealized Losses | 0 | 0
Fair Value | 388 | 372
Equity and other Securities: |  |
Amortized cost | 69,892 | 72,275
Gross Unrealized Gains | 886 | 871
Gross Unrealized Losses | 0 | 0
Equity and other securities | 70,778 | 73,146
U.S. Treasury and agency securities |  |
Available-for-Sale Portfolio: |  |
Amortized Cost | 2,382,653 | 2,381,168
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 327,136 | 300,385
Fair Value | 2,055,517 | 2,080,783
Held-to-Maturity Portfolio: |  |
Amortized Cost | 1,116,458 | 1,109,101
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 85,460 | 50,866
Fair Value | 1,030,998 | 1,058,235
Obligations of state and political subdivisions |  |
Available-for-Sale Portfolio: |  |
Amortized Cost | 496,983 | 502,879
Gross Unrealized Gains | 630 | 1,469
Gross Unrealized Losses | 37,464 | 29,985
Fair Value | 460,149 | 474,363
Government agency mortgage-backed securities |  |
Available-for-Sale Portfolio: |  |
Amortized Cost | 391,020 | 400,062
Gross Unrealized Gains | 46 | 76
Gross Unrealized Losses | 58,587 | 51,612
Fair Value | 332,479 | 348,526
Held-to-Maturity Portfolio: |  |
Amortized Cost | 101,014 | 63,073
Gross Unrealized Gains | 477 | 688
Gross Unrealized Losses | 874 | 180
Fair Value | 100,617 | 63,581
Corporate debt securities |  |
Available-for-Sale Portfolio: |  |
Amortized Cost | 8,000 | 8,000
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 560 | 606
Fair Value | 7,440 | 7,394
Government agency collateralized mortgage obligations |  |
Available-for-Sale Portfolio: |  |
Amortized Cost | 8,841 | 9,498
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 562 | 572
Fair Value | 8,279 | 8,926
Held-to-Maturity Portfolio: |  |
Amortized Cost | 0 |
Fair Value | 0 |
Federal Home Loan Bank Common Stock |  |
Other equity securities, at adjusted cost |  |
Amortized Cost | 29,866 | 32,526
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 29,866 | 32,526
Federal Reserve Bank Common Stock |  |
Other equity securities, at adjusted cost |  |
Amortized Cost | 33,568 | 33,568
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 33,568 | 33,568
Other Equity Securities |  |
Other equity securities, at adjusted cost |  |
Amortized Cost | 6,206 | 5,930
Gross Unrealized Gains | 750 | 750
Gross Unrealized Losses | 0 | 0
Fair Value | $ 6,956 | $ 6,680

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