Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor) (Detail) $ in Millions	Sep. 30, 2017 USD ($)
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	$ 1,803
Wrapped Securities [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	30
Gross unrealized losses	(5)
Loss and loss adjustment expense reserves	16	[1]
Asset-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	0	[2]
Gross unrealized losses	0	[2]
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	15	[2]
Gross unrealized losses	(4)	[2]
Loss and loss adjustment expense reserves	16	[1],[2]
Corporate Obligations [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	13	[2]
Gross unrealized losses	(1)	[2]
Loss and loss adjustment expense reserves	0	[1],[2]
All Other Securities [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	0	[2]
Gross unrealized losses	0	[2]
Loss and loss adjustment expense reserves	0	[1],[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	[1]

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

[2] Includes investments insured by MBIA Corp. and National.