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Securities (Narrative) (Detail) - USD ($) $ in Thousands | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | --- | ---
Schedule Of Marketable Securities [Line Items] |  |  |
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, pre tax | $ 2,500 | $ 3,500 |
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax | 2,000 | 2,800 |
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 $158 in 2021 and $160 in 2020 | $ 158 | 160 |
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 | 76.60% |  |
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 $158 in 2021 and $160 in 2020 | 158 | 160 | $ 0
Held-to-maturity Securities [Member] |  |  |
Schedule Of Marketable Securities [Line Items] |  |  |
Carry value of securities pledged | 642,300 | 659,200 |
Accrued interest receivable | 18,400 | 21,700 |
Reclassified To Held To Maturity [Member] |  |  |
Schedule Of Marketable Securities [Line Items] |  |  |
Securities available for sale, at estimated fair value |  |  | 377,800
Available for sale securities reclassified to held to maturity, unrealized gain |  |  | 3,300
Available for sale securities reclassified to held to maturity, unrealized gain net of tax |  |  | $ 2,600
Available-for-sale Securities [Member] |  |  |
Schedule Of Marketable Securities [Line Items] |  |  |
Carry value of securities pledged | 5,800,000 | 4,400,000 |
Accrued interest receivable | 120,500 | $ 111,000 |
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 $158 in 2021 and $160 in 2020 | 0 |  |
Pre-Refunded [Member] | Debt Securities [Member] |  |  |
Schedule Of Marketable Securities [Line Items] |  |  |
Securities held to maturity, net of allowance for credit losses of $158 in 2021 and $160 in 2020 | $ 0 |  |

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