Investments (Securities Held In Unrealized Loss Position and Insured By Financial Guarantor and the Related Insurance Loss Reserve On Company Insured Investments) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Schedule of Investments [Line Items]
Total, Fair Value	$ 1,025		$ 2,387
Total, Unrealized Losses	(191)		(478)
Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	458
Total, Unrealized Losses	(137)
Loss and Loss Adjustment Expense Reserves	18	[1]
Asset-Backed Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	273
Total, Unrealized Losses	(99)
Asset-Backed Securities [Member] | Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	175
Total, Unrealized Losses	(70)
Loss and Loss Adjustment Expense Reserves	18	[1]
Asset-Backed Securities [Member] | Wrapped Securities [Member] | Other Insurers [Member]
Schedule of Investments [Line Items]
Total, Fair Value	27
Total, Unrealized Losses	(7)
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	(63)	[2]
Loss and Loss Adjustment Expense Reserves	18	[1],[2]
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	270
Total, Unrealized Losses	(52)
Mortgage-Backed Securities [Member] | Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	194
Total, Unrealized Losses	(44)
Mortgage-Backed Securities [Member] | Wrapped Securities [Member] | Other Insurers [Member]
Schedule of Investments [Line Items]
Total, Fair Value	184
Total, Unrealized Losses	(43)
Mortgage-Backed Securities [Member] | Wrapped Securities [Member] | MBIA Corp and National [Member]
Schedule of Investments [Line Items]
Total, Fair Value	10	[2]
Total, Unrealized Losses	(1)	[2]
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Total, Fair Value	163
Total, Unrealized Losses	(28)
Corporate Obligations [Member] | Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	9
Total, Unrealized Losses	(12)
Corporate Obligations [Member] | Wrapped Securities [Member] | Other Insurers [Member]
Schedule of Investments [Line Items]
Total, Fair Value	9
Total, Unrealized Losses	(12)
Other Investments [Member] | Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	80
Total, Unrealized Losses	(11)
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	72	[2]
Total, Unrealized Losses	$ (11)	[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.