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Securities (Narrative) (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Schedule Of Marketable Securities [Line Items] |  |
Securities available for sale, at estimated fair value | $ 10,437,565 | $ 11,269,591
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, pre tax | 3,500 | 4,800
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax | 2,800 | 3,800
Debt Securities, Held-to-maturity, Past Due | 0 |
Debt Securities, Held-to-maturity, Allowance for Credit Loss | $ 160 | 0
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 | 77.80% |
Debt Securities, Available-for-sale, Allowance for Credit Loss | $ 0 | 0
Debt Securities [Member] |  |
Schedule Of Marketable Securities [Line Items] |  |
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 160 | 0
Held-to-maturity Securities [Member] |  |
Schedule Of Marketable Securities [Line Items] |  |
Carry value of securities pledged | 659,200 | 561,400
Accrued interest receivable | 21,700 | 21,100
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
Securities Available For Sale [Member] |  |
Schedule Of Marketable Securities [Line Items] |  |
Carry value of securities pledged | 4,400,000 | 3,400,000
Accrued interest receivable | 111,000 | $ 115,900
Guaranteed by the Texas PSF [Member] | Debt Securities [Member] |  |
Schedule Of Marketable Securities [Line Items] |  |
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0 |
Pre-Refunded [Member] | Debt Securities [Member] |  |
Schedule Of Marketable Securities [Line Items] |  |
Debt Securities, Held-to-maturity, Allowance for Credit Loss | $ 0 |

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