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Securities (Narrative) (Detail) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 | Dec. 31, 2018
Schedule Of Marketable Securities [Line Items] |  |
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas | 99.70% |
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund | 69.10% |
Carry value of securities pledged | $ 3,900,000 | $ 3,800,000
Securities available for sale, at estimated fair value | 11,269,591 | 11,387,321
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, pre tax | 4,800 | 2,700
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax | 3,800 | $ 2,200
Other than temporary impairment, credit losses recognized in earnings, credit losses on debt securities held | 0 |
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 |

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