Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor) (Detail) - Mortgage-backed [Member] - Mbia Corp And National [Member] - Financial Guarantee [Member] $ in Millions	Mar. 31, 2020 USD ($)	[1]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	$ 9
Gross unrealized losses	(6)
Loss and loss adjustment expense reserves	$ 22	[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.