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Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor) (Detail) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value $ 2,470 $ 1,284
Gross unrealized losses (185) (152)
Wrapped Securities [Member]
    
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value 306  
Gross unrealized losses (71)  
Loss and loss adjustment expense reserves 15 [1]  
Asset-backed [Member]
    
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value 215  
Gross unrealized losses (74)  
Asset-backed [Member] | Wrapped Securities [Member]
    
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value 157  
Gross unrealized losses (63)  
Loss and loss adjustment expense reserves 15 [1]  
Asset-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
    
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value 148 [2]  
Gross unrealized losses (58) [2]  
Loss and loss adjustment expense reserves 15 [1],[2]  
Asset-backed [Member] | Wrapped Securities [Member] | Other Insurers [Member]
    
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value 9  
Gross unrealized losses (5)  
Mortgage-backed [Member]
    
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value 611  
Gross unrealized losses (25)  
Mortgage-backed [Member] | Wrapped Securities [Member]
    
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value 20  
Gross unrealized losses 0  
Mortgage-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
    
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value 5 [2]  
Gross unrealized losses 0 [2]  
Mortgage-backed [Member] | Wrapped Securities [Member] | Other Insurers [Member]
    
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value 15  
Gross unrealized losses 0  
Corporate Obligations [Member]
    
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value 630  
Gross unrealized losses (26)  
Corporate Obligations [Member] | Wrapped Securities [Member]
    
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value 6  
Gross unrealized losses (2)  
Corporate Obligations [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
    
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value 0 [2]  
Gross unrealized losses 0 [2]  
Corporate Obligations [Member] | Wrapped Securities [Member] | Other Insurers [Member]
    
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value 6  
Gross unrealized losses (2)  
All Other Securities [Member] | Wrapped Securities [Member]
    
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value 123  
Gross unrealized losses (6)  
All Other Securities [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
    
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value 104 [2]  
Gross unrealized losses (6) [2]  
All Other Securities [Member] | Wrapped Securities [Member] | Other Insurers [Member]
    
Schedule Of Investments [Line Items]    
Total Available-For-Sale, Fair Value 19  
Gross unrealized losses $ 0  
[1] Insurance loss reserve estimates are based on the proportion of par value owned to the total amount of par value insured.
[2] Includes investments insured by MBIA Corp. and National.