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Securities - Additional Information (Detail) (USD $)
 3 Months Ended
Mar. 31, 2014
Dec. 31, 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 96.70%    
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund 68.80%    
Carrying value of pledged securities $ 2,600,000,000 $ 3,000,000,000  
Available for Sale, Estimated Fair Value 5,482,465,000 5,895,436,000  
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 unamortized unrealized gain on transferred securities included in accumulated other comprehensive income Pre Tax 120,200,000    
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax 78,100,000    
Reclassified To Held To Maturity [Member]
      
Schedule Of Marketable Securities [Line Items]      
Available for Sale, Estimated Fair Value     $ 2,300,000,000