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Securities (Narrative) (Detail) - USD ($) $ in Thousands | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
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Schedule Of Marketable Securities [Line Items] |  |  |  |
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, pre tax | $ 1,200 | $ 1,800 |  |
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax | 909 | 1,400 |  |
Debt Securities, Held-to-maturity, 90 Days or More Past Due, Still Accruing | 0 |  |  |
Securities held to maturity, net of allowance for credit losses of $310 in 2023 and $158 in 2022 | $ 310 | 158 |  |
Percent Municipal Bond Portfolio Issued By Political Subdivision Or Agencies Within State Of Texas Guaranteed By Texas Permanent School Fund Or Have Been Pre-Refunded | 73.50% |  |  |
Debt Securities, Available-for-sale, Allowance for Credit Loss | $ 0 | 0 |  |
Debt Securities [Member] |  |  |  |
Schedule Of Marketable Securities [Line Items] |  |  |  |
Securities held to maturity, net of allowance for credit losses of $310 in 2023 and $158 in 2022 | 310 | 158 | $ 158 | $ 160
Held-to-maturity Securities [Member] |  |  |  |
Schedule Of Marketable Securities [Line Items] |  |  |  |
Carry value of securities pledged | 1,000,000 | 256,300 |  |
Accrued interest receivable | 40,900 | 30,200 |  |
Guaranteed by the Texas PSF [Member] | Debt Securities [Member] |  |  |  |
Schedule Of Marketable Securities [Line Items] |  |  |  |
Securities held to maturity, net of allowance for credit losses of $310 in 2023 and $158 in 2022 | 0 |  |  |
Pre-Refunded [Member] | Debt Securities [Member] |  |  |  |
Schedule Of Marketable Securities [Line Items] |  |  |  |
Securities held to maturity, net of allowance for credit losses of $310 in 2023 and $158 in 2022 | 0 |  |  |
Available-for-sale Securities [Member] |  |  |  |
Schedule Of Marketable Securities [Line Items] |  |  |  |
Carry value of securities pledged | 6,100,000 | 8,000,000 |  |
Accrued interest receivable | $ 114,900 | $ 140,600 |  |

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