Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor) (Detail) $ in Millions	Sep. 30, 2018 USD ($)
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	$ 2,655
Wrapped Securities [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	92	[1]
Gross unrealized losses	(15)	[1]
Loss and loss adjustment expense reserves	14	[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	(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	70	[1]
Gross unrealized losses	(12)	[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	8	[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	2
Gross unrealized losses	0
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.