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Securities Available for Sale - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Parenthetical) (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Investment Securities [Line Items]    
Mortgage Backed Securities, Amortized Cost $ 1,169,613 $ 1,140,212
Mortgage-Backed Securities, Estimated Fair Value 1,171,303 1,142,776
Obligations of U.S. Government [Member]
   
Investment Securities [Line Items]    
Mortgage Backed Securities, Amortized Cost 28,000  
Mortgage-Backed Securities, Estimated Fair Value 32,000  
Obligations of U.S. Government-Sponsored Enterprises [Member]
   
Investment Securities [Line Items]    
Mortgage Backed Securities, Amortized Cost 839,000  
Mortgage-Backed Securities, Estimated Fair Value $ 864,000