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Securities - Additional Information (Detail) - USD ($) $ in Thousands | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2015 | Dec. 31, 2014 | 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 | 62.80% |  |
Securities Pledged as Collateral | $ 3,300,000 | $ 3,000,000 |
Available for Sale, Estimated Fair Value | 8,972,688 | $ 8,461,254 |
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 | 69,000 |  |
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax | $ 44,800 |  |
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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