Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	$ 1,360		$ 2,071
Wrapped Securities [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	128
Gross unrealized losses	(36)
Loss and loss adjustment expense reserves	49	[1]
Asset-backed [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	156
Gross unrealized losses	(34)
Asset-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	97	[2]
Gross unrealized losses	(33)	[2]
Loss and loss adjustment expense reserves	38	[1],[2]
Mortgage-backed [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	388
Gross unrealized losses	(8)
Mortgage-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	7	[2]
Gross unrealized losses	0	[2]
Loss and loss adjustment expense reserves	11	[1],[2]
Corporate Obligations [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	457
Gross unrealized losses	(103)
All Other Securities [Member] | Wrapped Securities [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	24
Gross unrealized losses	(3)
Loss and loss adjustment expense reserves	0	[1]
All Other Securities [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	23	[2]
Gross unrealized losses	(3)	[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	1
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.