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Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor) (Detail)
$ in Millions
Mar. 31, 2018
USD ($)
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value $ 2,688
Wrapped Securities [Member]  
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value 74 [1]
Gross unrealized losses (4) [1]
Loss and loss adjustment expense reserves 14 [2]
Asset-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]  
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value 0 [1]
Gross unrealized losses 0 [1]
Loss and loss adjustment expense reserves 0 [2]
Mortgage-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]  
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value 16 [1]
Gross unrealized losses (3) [1]
Loss and loss adjustment expense reserves 14 [2]
Corporate Obligations [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]  
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value 51 [1]
Gross unrealized losses (1) [1]
Loss and loss adjustment expense reserves 0 [2]
All Other Securities [Member] | Wrapped Securities [Member]  
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value 7 [1]
Gross unrealized losses 0 [1]
Loss and loss adjustment expense reserves 0 [2]
All Other Securities [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]  
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value 6 [1]
Gross unrealized losses 0 [1]
Loss and loss adjustment expense reserves 0 [2]
All Other Securities [Member] | Wrapped Securities [Member] | Other Insurers [Member]  
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value 1 [1]
Gross unrealized losses 0 [1]
Loss and loss adjustment expense reserves $ 0 [2]
[1]
Includes investments insured by MBIA Corp. and National.
[2]
Insurance loss reserve estimates are based on the proportion of par value owned to the total amount of par value insured.