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Investments in fixed maturity securities - Amortized cost and estimated fair value of securities with fixed maturities (Detail) (Fixed Maturities [Member], USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Summary of Investment Holdings [Line Items]    
Due in one year or less - amortized cost $ 6,633  
Due after one year through five years - amortized cost 12,839  
Due after five years through ten years - amortized cost 4,531  
Due after ten years - amortized cost 2,437  
Total - amortized cost 28,568  
Due in one year or less - fair value 6,818  
Due after one year through five years - fair value 13,937  
Due after five years through ten years - fair value 5,298  
Due after ten years - fair value 2,902  
Fair Value of Investments 31,383 32,291
Mortgage-backed securities [Member]
   
Summary of Investment Holdings [Line Items]    
Mortgage-backed securities - amortized cost 2,128  
Mortgage-backed securities - fair value 2,428  
Fair Value of Investments $ 2,428 $ 2,587