Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor) (Detail) - USD ($) $ in Millions		Mar. 31, 2016	Dec. 31, 2015
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		$ 1,206	$ 2,802
Wrapped Securities [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		141
Gross unrealized losses		(32)
Loss and loss adjustment expense reserves	[1]	28
Asset-backed [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		240
Gross unrealized losses		(30)
Asset-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	[2]	54
Gross unrealized losses	[2]	(28)
Loss and loss adjustment expense reserves	[1],[2]	13
Mortgage-backed [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		379
Gross unrealized losses		(11)
Mortgage-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	[2]	18
Gross unrealized losses	[2]	(2)
Loss and loss adjustment expense reserves	[1],[2]	15
Corporate Obligations [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		386
Gross unrealized losses		(96)
All Other Securities [Member] | Wrapped Securities [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		69
Gross unrealized losses		(2)
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]	67
Gross unrealized losses	[2]	(2)
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.