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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