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Securities - Additional Information (Detail) - USD ($) $ in Thousands | Jun. 30, 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 | $ 3,000 | $ 3,500
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax | $ 2,400 | 2,800
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 | 74.10% |
Debt Securities, Held-to-maturity, Allowance for Credit Loss | $ 0 | 0
Held-to-maturity Securities [Member] |  |
Schedule Of Marketable Securities [Line Items] |  |
Carry value of securities pledged | 530,500 | 659,200
Accrued interest receivable | 19,500 | 21,700
Available-for-sale Securities [Member] |  |
Schedule Of Marketable Securities [Line Items] |  |
Carry value of securities pledged | 4,600,000 | 4,400,000
Accrued interest receivable | $ 111,300 | $ 111,000

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