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Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Jun. 30, 2024 | Dec. 31, 2023 | Sep. 30, 2023 | Jun. 30, 2023 | Dec. 31, 2022
--- | --- | --- | --- | --- | --- | ---
Held-to-maturity |  |  |  |  |  |
Amortized Cost | $ 3,661,914 |  | $ 3,729,502 |  |  |
Allowance for Credit Losses | (3,214) | $ (3,214) | (3,214) | $ (3,214) | $ (3,214) | $ (1,388)
Net Carrying Amount | 3,658,700 |  | 3,726,288 |  |  |
Gross Unrealized Gains | 747 |  | 638 |  |  |
Gross Unrealized (Losses) | (549,837) |  | (591,556) |  |  |
Estimated Fair Value | 3,109,610 |  | 3,135,370 |  |  |
Available-for-sale |  |  |  |  |  |
Amortized Cost | 2,976,811 |  | 3,509,709 |  |  |
Allowance for Credit Losses | 0 |  | 0 | (1,196) | (2,396) | 0
Gross Unrealized Gains | 380 |  | 230 |  |  |
Gross Unrealized (Losses) | (286,097) |  | (357,786) |  |  |
Estimated Fair Value | 2,691,094 |  | 3,152,153 |  |  |
U.S. Treasury |  |  |  |  |  |
Available-for-sale |  |  |  |  |  |
Amortized Cost | 1,297 |  | 2,285 |  |  |
Allowance for Credit Losses | 0 |  | 0 |  |  |
Gross Unrealized Gains | 0 |  | 0 |  |  |
Gross Unrealized (Losses) | (7) |  | (31) |  |  |
Estimated Fair Value | 1,290 |  | 2,254 |  |  |
U.S. Government agencies |  |  |  |  |  |
Held-to-maturity |  |  |  |  |  |
Amortized Cost | 455,179 |  | 453,121 |  |  |
Allowance for Credit Losses | 0 |  | 0 |  |  |
Net Carrying Amount | 455,179 |  | 453,121 |  |  |
Gross Unrealized Gains | 0 |  | 0 |  |  |
Gross Unrealized (Losses) | (75,331) |  | (89,203) |  |  |
Estimated Fair Value | 379,848 |  | 363,918 |  |  |
Available-for-sale |  |  |  |  |  |
Amortized Cost | 59,247 |  | 74,460 |  |  |
Allowance for Credit Losses | 0 |  | 0 |  |  |
Gross Unrealized Gains | 23 |  | 35 |  |  |
Gross Unrealized (Losses) | (873) |  | (1,993) |  |  |
Estimated Fair Value | 58,397 |  | 72,502 |  |  |
Mortgage-backed securities |  |  |  |  |  |
Held-to-maturity |  |  |  |  |  |
Amortized Cost | 1,093,070 |  | 1,161,694 |  |  |
Allowance for Credit Losses | 0 |  | 0 |  |  |
Net Carrying Amount | 1,093,070 |  | 1,161,694 |  |  |
Gross Unrealized Gains | 571 |  | 354 |  |  |
Gross Unrealized (Losses) | (88,660) |  | (107,834) |  |  |
Estimated Fair Value | 1,004,981 |  | 1,054,214 |  |  |
Available-for-sale |  |  |  |  |  |
Amortized Cost | 1,634,391 |  | 2,138,652 |  |  |
Allowance for Credit Losses | 0 |  | 0 |  |  |
Gross Unrealized Gains | 18 |  | 8 |  |  |
Gross Unrealized (Losses) | (124,007) |  | (198,353) |  |  |
Estimated Fair Value | 1,510,402 |  | 1,940,307 |  |  |
State and political subdivisions |  |  |  |  |  |
Held-to-maturity |  |  |  |  |  |
Amortized Cost | 1,857,510 |  | 1,858,680 |  |  |
Allowance for Credit Losses | (227) | (191) | (2,006) | (1,520) | (934) | (110)
Net Carrying Amount | 1,857,283 |  | 1,856,674 |  |  |
Gross Unrealized Gains | 176 |  | 284 |  |  |
Gross Unrealized (Losses) | (368,773) |  | (369,509) |  |  |
Estimated Fair Value | 1,488,686 |  | 1,487,449 |  |  |
Available-for-sale |  |  |  |  |  |
Amortized Cost | 1,042,823 |  | 1,035,147 |  |  |
Allowance for Credit Losses | 0 |  | 0 | 0 | 0 | 0
Gross Unrealized Gains | 180 |  | 187 |  |  |
Gross Unrealized (Losses) | (144,825) |  | (132,541) |  |  |
Estimated Fair Value | 898,178 |  | 902,793 |  |  |
Other securities |  |  |  |  |  |
Held-to-maturity |  |  |  |  |  |
Amortized Cost | 256,155 |  | 256,007 |  |  |
Allowance for Credit Losses | (2,987) | $ (3,023) | (1,208) | (1,694) | (2,280) | (1,278)
Net Carrying Amount | 253,168 |  | 254,799 |  |  |
Gross Unrealized Gains | 0 |  | 0 |  |  |
Gross Unrealized (Losses) | (17,073) |  | (25,010) |  |  |
Estimated Fair Value | 236,095 |  | 229,789 |  |  |
Available-for-sale |  |  |  |  |  |
Amortized Cost | 239,053 |  | 259,165 |  |  |
Allowance for Credit Losses | 0 |  | 0 | $ (1,196) | $ (2,396) | $ 0
Gross Unrealized Gains | 159 |  | 0 |  |  |
Gross Unrealized (Losses) | (16,385) |  | (24,868) |  |  |
Estimated Fair Value | $ 222,827 |  | $ 234,297 |  |  |

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