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	Mar. 31, 2013
Schedule of Investments [Line Items]
Total, Fair Value	$ 1,130
Total, Unrealized Losses	(126)
Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	329
Total, Unrealized Losses	(86)
Loss and Loss Adjustment Expense Reserves	16	[1]
Asset-Backed Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	227
Total, Unrealized Losses	(82)
Asset-Backed Securities [Member] | Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	155
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	146	[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	551
Total, Unrealized Losses	(18)
Mortgage-Backed Securities [Member] | Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	88
Total, Unrealized Losses	(7)
Mortgage-Backed Securities [Member] | Wrapped Securities [Member] | Other Insurers [Member]
Schedule of Investments [Line Items]
Total, Fair Value	82
Total, Unrealized Losses	(7)
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	352
Total, Unrealized Losses	(26)
Corporate Obligations [Member] | Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	11
Total, Unrealized Losses	(10)
Corporate Obligations [Member] | Wrapped Securities [Member] | Other Insurers [Member]
Schedule of Investments [Line Items]
Total, Fair Value	11
Total, Unrealized Losses	(10)
Other Investments [Member] | Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	75
Total, Unrealized Losses	(6)
Other Investments [Member] | Wrapped Securities [Member] | Other Insurers [Member]
Schedule of Investments [Line Items]
Total, Fair Value	7
Other Investments [Member] | Wrapped Securities [Member] | MBIA Corp And National [Member]
Schedule of Investments [Line Items]
Total, Fair Value	68	[2]
Total, Unrealized Losses	$ (6)	[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.