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Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Parenthetical) (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Investment Securities [Line Items]    
Amortized Cost $ 1,123,728 $ 1,140,212
Estimated fair Value of Available for Sale 1,130,822 1,142,776
Obligations of U.S. Government Agencies [Member]
   
Investment Securities [Line Items]    
Amortized Cost 29,200  
Estimated fair Value of Available for Sale 33,400  
Obligations of U.S. Government-Sponsored Enterprises [Member]
   
Investment Securities [Line Items]    
Amortized Cost 787,100  
Estimated fair Value of Available for Sale $ 818,400