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Investments (details) - Net Investment Income (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Net investment income      
Gross investment income $ 2,910 $ 3,093 $ 2,809
Fixed maturities [Member]
     
Net investment income      
Gross investment income 2,543 2,710 2,822
Equity securities [Member]
     
Net investment income      
Gross investment income 29 31 30
Short-term securities [Member]
     
Net investment income      
Gross investment income 12 13 27
Real estate [Member]
     
Net investment income      
Gross investment income 34 35 36
Other investments [Member]
     
Net investment income      
Gross investment income $ 292 $ 304 $ (106)