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Available for Sale Securities (Unrealized Loss Position) (Detail) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Sep. 30, 2011
Schedule of Available-for-sale Securities [Line Items]      
Fair value, less than 12 months   $ 1 $ 1
Gross unrealized losses, less than 12 months         
Gross unrealized losses, 12 months or longer         
Fair Value   1 1
Gross unrealized losses, Total         
Held-to-maturity Securities, Gross unrealized losses, less than 12 months        
Held-to-maturity Securities, Collateralized mortgage obligations Fair Value 12 months or longer        
Held-to-maturity Securities, Gross unrealized losses, 12 months or longer        
Held-to-maturity Securities, Gross unrealized losses        
Collateralized mortgage obligations [Member]
     
Schedule of Available-for-sale Securities [Line Items]      
Fair value, less than 12 months 14 3 5
Gross unrealized losses, less than 12 months         
Fair value, 12 months or longer 3    
Gross unrealized losses, 12 months or longer         
Fair Value 17 3 5
Gross unrealized losses, Total         
Held-to-maturity Securities, Collateralized mortgage obligations Fair Value less than 12 months     404
Held-to-maturity Securities, Gross unrealized losses, less than 12 months        
Held-to-maturity Securities, Collateralized mortgage obligations Fair Value 12 months or longer        
Held-to-maturity Securities, Gross unrealized losses, 12 months or longer        
Held-to-maturity Securities, Collateralized mortgage obligations Fair Value     404
Held-to-maturity Securities, Gross unrealized losses        
States and political subdivisions [Member]
     
Schedule of Available-for-sale Securities [Line Items]      
Fair value, less than 12 months 7   1
Gross unrealized losses, less than 12 months        
Gross unrealized losses, 12 months or longer        
Fair Value 7   1
Gross unrealized losses, Total        
Other securities [Member]
     
Schedule of Available-for-sale Securities [Line Items]      
Fair value, less than 12 months 21 4 411
Gross unrealized losses, less than 12 months         
Fair value, 12 months or longer 3    
Gross unrealized losses, 12 months or longer         
Fair Value 24 4 411
Gross unrealized losses, Total