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Securities - Additional Information (Detail) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
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Schedule Of Marketable Securities [Line Items] |  |
Securities available for sale, estimated fair value | $ 10,604,480 | $ 11,269,591
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income Pre Tax | 3,800 | 4,800
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax | $ 3,000 | 3,800
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 | 75.80% |
Held-to-maturity Securities [Member] |  |
Schedule Of Marketable Securities [Line Items] |  |
Carry value of securities pledged | $ 663,200 | 561,400
Accrued interest receivable | 11,900 | 21,100
Reclassified To Held To Maturity [Member] |  |
Schedule Of Marketable Securities [Line Items] |  |
Securities available for sale, 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 | 3,800,000 | 3,400,000
Accrued interest receivable | $ 66,600 | $ 115,900

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