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Securities - Additional Information (Detail) (USD $) | 6 Months Ended |
--- | --- | ---
Jun. 30, 2013 | 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 | 95.90% |
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund | 76.20% |
Carrying value of pledged securities | $ 2,300,000,000 | $ 2,700,000,000
Securities reclassified from available for sale to held to maturity | 6,033,370,000 | 6,203,299,000
Available for sale securities reclassified to held to maturity, unrealized gain |  | 165,700,000
Available for sale securities reclassified to held to maturity, unrealized gain net of tax |  | 107,700,000
Net unrealized gain on transferred securities included in accumulated other comprehensive income before Tax | 146,800,000 |
Net unrealized gain on transferred securities included in accumulated other comprehensive income after Tax | 95,400,000 |
Reclassified To Held To Maturity [Member] |  |
Schedule Of Marketable Securities [Line Items] |  |
Securities reclassified from available for sale to held to maturity |  | $ 2,300,000,000

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