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Securities (Narrative) (Detail) - USD ($) $ in Thousands | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2012
Schedule Of Marketable Securities [Line Items] |  |  |
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas | 98.20% |  |
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund | 67.80% |  |
Securities pledged as collateral | $ 3,800,000 | $ 3,900,000 |
Securities available for sale, at estimated fair value | 10,489,009 | 10,203,277 |
Available for sale securities reclassified to held to maturity, unrealized gain |  |  | $ 165,700
Available for sale securities reclassified to held to maturity, unrealized gain net of tax |  |  | 107,700
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, pre tax | 11,600 | 27,700 |
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax | 7,500 | $ 18,000 |
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 |  |  | $ 2,300,000

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