Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor) (Detail) - Financial Guarantee [Member] $ in Millions	Dec. 31, 2020 USD ($)
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	$ 3
Gross unrealized losses	0
Loss and loss adjustment expense reserves	1	[1]
Mortgage-backed [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	3
Gross unrealized losses	0
Loss and loss adjustment expense reserves	$ 1	[1]

[1] Insurance loss reserve estimates are based on the proportion of par value owned to the total amount of par value insured.