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Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor) (Detail)
$ in Millions
Dec. 31, 2017
USD ($)
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value $ 1,821
Wrapped Securities [Member]  
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value 124 [1]
Gross unrealized losses (11) [1]
Loss and loss adjustment expense reserves 49 [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 [1],[2]
Mortgage-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]  
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value 14 [1]
Gross unrealized losses (4) [1]
Loss and loss adjustment expense reserves 15 [2]
Corporate Obligations [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]  
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value 14 [1]
Gross unrealized losses 0 [1]
Loss and loss adjustment expense reserves 0 [1],[2]
All Other Securities [Member] | Wrapped Securities [Member]  
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value 96 [1]
Gross unrealized losses (7) [1]
Loss and loss adjustment expense reserves 34 [2]
All Other Securities [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]  
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value 93 [1]
Gross unrealized losses (7) [1]
Loss and loss adjustment expense reserves 34 [2]
All Other Securities [Member] | Wrapped Securities [Member] | Other Insurers [Member]  
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value 3 [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.