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Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023 | Mar. 31, 2023 | Dec. 31, 2022
--- | --- | --- | --- | ---
Held-to-maturity |  |  |  |
Amortized Cost | $ 3,710,472 | $ 3,729,502 |  |
Allowance for credit losses on HTM | (3,214) | (3,214) | $ (1,888) | $ (1,388)
Net Carrying Amount | 3,707,258 | 3,726,288 |  |
Gross Unrealized Gains | 324 | 638 |  |
Gross Unrealized (Losses) | (658,300) | (591,556) |  |
Estimated Fair Value | 3,049,282 | 3,135,370 |  |
Available-for-sale |  |  |  |
Amortized Cost | 3,394,971 | 3,509,709 |  |
Allowance for Credit Losses | 0 | 0 | (5,800) | 0
Gross Unrealized Gains | 232 | 230 |  |
Gross Unrealized (Losses) | (367,645) | (357,786) |  |
Estimated Fair Value | 3,027,558 | 3,152,153 |  |
U.S. Government agencies |  |  |  |
Held-to-maturity |  |  |  |
Amortized Cost | 453,805 | 453,121 |  |
Allowance for credit losses on HTM | 0 | 0 |  |
Net Carrying Amount | 453,805 | 453,121 |  |
Gross Unrealized Gains | 0 | 0 |  |
Gross Unrealized (Losses) | (94,887) | (89,203) |  |
Estimated Fair Value | 358,918 | 363,918 |  |
Available-for-sale |  |  |  |
Amortized Cost | 71,768 | 74,460 |  |
Allowance for Credit Losses | 0 | 0 |  |
Gross Unrealized Gains | 28 | 35 |  |
Gross Unrealized (Losses) | (1,995) | (1,993) |  |
Estimated Fair Value | 69,801 | 72,502 |  |
U.S. Treasury |  |  |  |
Available-for-sale |  |  |  |
Amortized Cost | 1,990 | 2,285 |  |
Allowance for Credit Losses | 0 | 0 |  |
Gross Unrealized Gains | 0 | 0 |  |
Gross Unrealized (Losses) | (26) | (31) |  |
Estimated Fair Value | 1,964 | 2,254 |  |
Mortgage-backed securities |  |  |  |
Held-to-maturity |  |  |  |
Amortized Cost | 1,142,352 | 1,161,694 |  |
Allowance for credit losses on HTM | 0 | 0 |  |
Net Carrying Amount | 1,142,352 | 1,161,694 |  |
Gross Unrealized Gains | 238 | 354 |  |
Gross Unrealized (Losses) | (126,553) | (107,834) |  |
Estimated Fair Value | 1,016,037 | 1,054,214 |  |
Available-for-sale |  |  |  |
Amortized Cost | 2,048,895 | 2,138,652 |  |
Allowance for Credit Losses | 0 | 0 |  |
Gross Unrealized Gains | 3 | 8 |  |
Gross Unrealized (Losses) | (203,534) | (198,353) |  |
Estimated Fair Value | 1,845,364 | 1,940,307 |  |
State and political subdivisions |  |  |  |
Held-to-maturity |  |  |  |
Amortized Cost | 1,857,894 | 1,858,680 |  |
Allowance for credit losses on HTM | (2,252) | (2,006) | (362) | (110)
Net Carrying Amount | 1,855,642 | 1,856,674 |  |
Gross Unrealized Gains | 86 | 284 |  |
Gross Unrealized (Losses) | (410,749) | (369,509) |  |
Estimated Fair Value | 1,444,979 | 1,487,449 |  |
Available-for-sale |  |  |  |
Amortized Cost | 1,013,190 | 1,035,147 |  |
Allowance for Credit Losses | 0 | 0 | 0 | 0
Gross Unrealized Gains | 142 | 187 |  |
Gross Unrealized (Losses) | (138,483) | (132,541) |  |
Estimated Fair Value | 874,849 | 902,793 |  |
Other securities |  |  |  |
Held-to-maturity |  |  |  |
Amortized Cost | 256,421 | 256,007 |  |
Allowance for credit losses on HTM | (962) | (1,208) | (1,526) | (1,278)
Net Carrying Amount | 255,459 | 254,799 |  |
Gross Unrealized Gains | 0 | 0 |  |
Gross Unrealized (Losses) | (26,111) | (25,010) |  |
Estimated Fair Value | 229,348 | 229,789 |  |
Available-for-sale |  |  |  |
Amortized Cost | 259,128 | 259,165 |  |
Allowance for Credit Losses | 0 | 0 | $ (5,800) | $ 0
Gross Unrealized Gains | 59 | 0 |  |
Gross Unrealized (Losses) | (23,607) | (24,868) |  |
Estimated Fair Value | $ 235,580 | $ 234,297 |  |

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