Investments (Securities Held In Unrealized Loss Position and Insured By Financial Guarantor and Related Insurance Loss Reserve On Company Insured Investments) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Schedule of Investments [Line Items]
Total, Fair Value	$ 880
Total, Unrealized Losses	(139)
Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	363
Total, Unrealized Losses	(101)
Loss and loss adjustment expense reserves	16	[1]
Asset-Backed Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	235
Total, Unrealized Losses	(90)
Asset-Backed Securities [Member] | Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	157
Total, Unrealized Losses	(63)
Loss and loss adjustment expense reserves	16	[1]
Asset-Backed Securities [Member] | Wrapped Securities [Member] | Other Insurers [Member]
Schedule of Investments [Line Items]
Total, Fair Value	9
Total, Unrealized Losses	(5)
Asset-Backed Securities [Member] | Wrapped Securities [Member] | MBIA Corp and National [Member]
Schedule of Investments [Line Items]
Total, Fair Value	148	[2]
Total, Unrealized Losses	(58)	[2]
Loss and loss adjustment expense reserves	16	[1],[2]
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	355
Total, Unrealized Losses	(23)
Mortgage-Backed Securities [Member] | Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	117
Total, Unrealized Losses	(15)
Mortgage-Backed Securities [Member] | Wrapped Securities [Member] | Other Insurers [Member]
Schedule of Investments [Line Items]
Total, Fair Value	111
Total, Unrealized Losses	(15)
Mortgage-Backed Securities [Member] | Wrapped Securities [Member] | MBIA Corp and National [Member]
Schedule of Investments [Line Items]
Total, Fair Value	6	[2]
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Total, Fair Value	290
Total, Unrealized Losses	(26)
Corporate Obligations [Member] | Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	10
Total, Unrealized Losses	(11)
Corporate Obligations [Member] | Wrapped Securities [Member] | Other Insurers [Member]
Schedule of Investments [Line Items]
Total, Fair Value	10
Total, Unrealized Losses	(11)
Other Investments [Member] | Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	79
Total, Unrealized Losses	(12)
Other Investments [Member] | Wrapped Securities [Member] | Other Insurers [Member]
Schedule of Investments [Line Items]
Total, Fair Value	8
Other Investments [Member] | Wrapped Securities [Member] | MBIA Corp and National [Member]
Schedule of Investments [Line Items]
Total, Fair Value	71	[2]
Total, Unrealized Losses	$ (12)	[2]

[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.