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Investments (Narrative) (Detail) (USD $)
In Millions, unless otherwise specified
 9 Months Ended 12 Months Ended
Sep. 30, 2013
security
Dec. 31, 2012
security
Schedule Of Investments [Line Items]    
Fair value of securities on deposit with various regulatory authorities $ 11 $ 10
Fair value of securities pledged as collateral 750 820
Cash pledged as collateral under investment agreements $ 24 $ 144
Weighted average contractual maturity period in years for securities in an unrealized loss position 19 years 23 years
Number of Securities in unrealized loss position 64 153
Rate that a security's fair value is below book value 5.00%  
Greater Than Five Percent [Member]
    
Schedule Of Investments [Line Items]    
Number of Securities in unrealized loss position 39 89