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Securities - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2016 | Dec. 31, 2015 | 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 | 97.70% |  |
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund | 68.00% |  |
Securities Pledged as Collateral | $ 3,400,000 | $ 3,900,000 |
Available for Sale, Estimated Fair Value | 9,067,229 | $ 9,206,358 |
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 | 51,800 |  |
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax | $ 33,700 |  |
Reclassified To Held To Maturity [Member] |  |  |
Schedule Of Marketable Securities [Line Items] |  |  |
Available for Sale, Estimated Fair Value |  |  | $ 2,300,000

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