Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor) (Detail) - USD ($) $ in Millions		Mar. 31, 2017	Dec. 31, 2016
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		$ 2,323	$ 2,172
Wrapped Securities [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		78
Gross unrealized losses		(5)
Loss and loss adjustment expense reserves	[1]	16
Asset-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	[2]	13
Gross unrealized losses	[2]	0
Loss and loss adjustment expense reserves	[1],[2]	0
Mortgage-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	[2]	16
Gross unrealized losses	[2]	(3)
Loss and loss adjustment expense reserves	[1],[2]	16
Corporate Obligations [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	[2]	41
Gross unrealized losses	[2]	(2)
Loss and loss adjustment expense reserves	[1],[2]	0
All Other Securities [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	[2]	6
Gross unrealized losses	[2]	0
Loss and loss adjustment expense reserves	[1],[2]	0
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	[1]	$ 0

[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.