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Gross Unrealized Losses and Fair Values for Investments in an Unrealized Loss Position (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2011
Dec. 31, 2010
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months, Fair Value $ 3,178 $ 6,883
Less than 12 Months, Unrealized Loss (111) (125)
12 Months or Greater, Fair Value 42  
12 Months or Greater, Unrealized Loss (3)  
Total Fair Value 3,220  
Total Unrealized Loss (114)  
U.S. government agencies
    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months, Fair Value   831
Less than 12 Months, Unrealized Loss   (8)
U.S. government notes
    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months, Fair Value   2,225
Less than 12 Months, Unrealized Loss   (50)
Foreign government bonds
    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months, Fair Value 302 129
Less than 12 Months, Unrealized Loss (11) (5)
12 Months or Greater, Fair Value 6  
12 Months or Greater, Unrealized Loss 0  
Total Fair Value 308  
Total Unrealized Loss (11)  
Municipal securities
    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months, Fair Value   962
Less than 12 Months, Unrealized Loss   (5)
Corporate debt securities
    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months, Fair Value 2,160 1,061
Less than 12 Months, Unrealized Loss (97) (25)
12 Months or Greater, Fair Value 17  
12 Months or Greater, Unrealized Loss (1)  
Total Fair Value 2,177  
Total Unrealized Loss (98)  
Agency residential mortgage-backed securities
    
Investments, Unrealized Loss Position [Line Items]    
Less than 12 Months, Fair Value 716 1,675
Less than 12 Months, Unrealized Loss (3) (32)
12 Months or Greater, Fair Value 19  
12 Months or Greater, Unrealized Loss (2)  
Total Fair Value 735  
Total Unrealized Loss $ (5)