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Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor) (Detail) - Financial Guarantee [Member]
$ in Millions
Sep. 30, 2021
USD ($)
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value $ 38
Gross unrealized losses (2)
Loss and loss adjustment expense reserves 0 [1]
Mortgage-backed [Member]  
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value 1
Gross unrealized losses 0
Loss and loss adjustment expense reserves 0 [1]
Corporate Obligations [Member]  
Schedule Of Investments [Line Items]  
Total Available-For-Sale, Fair Value 37
Gross unrealized losses (2)
Loss and loss adjustment expense reserves $ 0 [1]
[1] Insurance loss reserve estimates are based on the proportion of par value owned to the total amount of par value insured and are discounted using a discount rate equal to the risk-free rate applicable to the currency and weighted average remaining life of the insurance contract and may differ from the fair value.