Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor) (Detail) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		$ 2,802	$ 2,071
Wrapped Securities [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		153
Gross unrealized losses		(38)
Loss and loss adjustment expense reserves	[1]	28
Asset-backed [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		307
Gross unrealized losses		(35)
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]	(33)
Loss and loss adjustment expense reserves	[1],[2]	14
Mortgage-backed [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		625
Gross unrealized losses		(15)
Mortgage-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	[2]	19
Gross unrealized losses	[2]	(2)
Loss and loss adjustment expense reserves	[1],[2]	14
Corporate Obligations [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		771
Gross unrealized losses		(103)
Corporate Obligations [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	[2]	0
Gross unrealized losses	[2]	0
Loss and loss adjustment expense reserves	[1],[2]	0
All Other Securities [Member] | Wrapped Securities [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		80
Gross unrealized losses		(3)
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]	80
Gross unrealized losses	[2]	(3)
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.