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INVESTMENT SECURITIES - Amortized cost and estimated fair value of investment securities (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2024 | Dec. 31, 2023
Available-for-Sale Portfolio: |  |
Amortized Cost | $ 3,203,694 | $ 3,301,607
Gross Unrealized Gains | 994 | 1,545
Gross Unrealized Losses | 318,745 | 383,160
Fair Value | 2,885,943 | 2,919,992
Held-to-Maturity Portfolio: |  |
Amortized Cost | 1,319,050 | 1,172,174
Gross Unrealized Gains | 1,817 | 688
Gross Unrealized Losses | 44,191 | 51,046
Fair Value | 1,276,676 | 1,121,816
Equity and other Securities: |  |
Equity and other securities | 82,558 | 73,146
Other equity securities, at adjusted cost |  |
Equity securities, Fair values |  | 400
Equity securities without readily determinable fair value amount | 80,400 | 72,800
U.S. Treasury and agency securities |  |
Available-for-Sale Portfolio: |  |
Amortized Cost | 2,386,644 | 2,381,168
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 244,022 | 300,385
Fair Value | 2,142,622 | 2,080,783
Held-to-Maturity Portfolio: |  |
Amortized Cost | 1,131,278 | 1,109,101
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 43,848 | 50,866
Fair Value | 1,087,430 | 1,058,235
Obligations of state and political subdivisions |  |
Available-for-Sale Portfolio: |  |
Amortized Cost | 430,699 | 502,879
Gross Unrealized Gains | 878 | 1,469
Gross Unrealized Losses | 29,625 | 29,985
Fair Value | 401,952 | 474,363
Government agency mortgage-backed securities |  |
Available-for-Sale Portfolio: |  |
Amortized Cost | 370,845 | 400,062
Gross Unrealized Gains | 116 | 76
Gross Unrealized Losses | 44,452 | 51,612
Fair Value | 326,509 | 348,526
Held-to-Maturity Portfolio: |  |
Amortized Cost | 187,772 | 63,073
Gross Unrealized Gains | 1,817 | 688
Gross Unrealized Losses | 343 | 180
Fair Value | 189,246 | 63,581
Corporate debt securities |  |
Available-for-Sale Portfolio: |  |
Amortized Cost | 8,000 | 8,000
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 327 | 606
Fair Value | 7,673 | 7,394
Government agency collateralized mortgage obligations |  |
Available-for-Sale Portfolio: |  |
Amortized Cost | 7,506 | 9,498
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 319 | 572
Fair Value | 7,187 | 8,926
Held-to-Maturity Portfolio: |  |
Amortized Cost | 0 |
Fair Value | 0 |
Federal Home Loan Bank Common Stock |  |
Other equity securities, at adjusted cost |  |
Equity securities without readily determinable fair value amount | 40,700 | 32,500
Federal Reserve Bank Common Stock |  |
Other equity securities, at adjusted cost |  |
Equity securities, Fair values | 2,100 |
Equity securities without readily determinable fair value amount | 33,400 | 33,600
Other Equity Securities |  |
Other equity securities, at adjusted cost |  |
Equity securities without readily determinable fair value amount | $ 6,300 | $ 6,700

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