Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor) (Detail) - USD ($) $ in Millions		Sep. 30, 2016	Dec. 31, 2015
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		$ 1,008	$ 2,802
Wrapped Securities [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		74
Gross unrealized losses		(11)
Loss and loss adjustment expense reserves	[1]	19
Asset-backed [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		166
Gross unrealized losses		(7)
Asset-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	[2]	49
Gross unrealized losses	[2]	(7)
Loss and loss adjustment expense reserves	[1],[2]	3
Mortgage-backed [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		280
Gross unrealized losses		(7)
Mortgage-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	[2]	17
Gross unrealized losses	[2]	(3)
Loss and loss adjustment expense reserves	[1],[2]	16
Corporate Obligations [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		270
Gross unrealized losses		(81)
All Other Securities [Member] | Wrapped Securities [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		8
Gross unrealized losses		(1)
Loss and loss adjustment expense reserves	[1]	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]	8
Gross unrealized losses	[2]	(1)
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		0
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.