Business Segments (Summary Of Company's Segment Results) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Dec. 31, 2012
Segment Reporting Information [Line Items]
Net gains (losses) on extinguishment of debt	$ 4
Total revenues	219		383
Total expenses	4		362
Income (loss) before income taxes	215		21
Identifiable assets	21,592		26,126		21,724
Losses and loss adjustment	(194)
Non Variable Interest Entities [Member]
Segment Reporting Information [Line Items]
Net gains (losses) on extinguishment of debt	4
Revenues	164	[1]	206	[1]
Net change in fair value of insured derivatives	(61)		299
Net gains (losses) on financial instruments at fair value and foreign exchange	63		(19)
Net investment losses related to other-than-temporary impairments			(94)
Losses and loss adjustment	(194)		97
Other net realized gains (losses)			1
Operating	122		171
Interest	60		73
Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Revenues of consolidated VIEs	49		(10)
Expenses of consolidated VIEs	16		21
Identifiable assets	8,300				8,300
Net gains (losses) on financial instruments at fair value and foreign exchange	33		(27)
Interest	12		15
U.S. Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	46	[2]	36	[2]
Total revenues	186		171
Inter-segment expenses	26	[2]	27	[2]
Total expenses	44		116
Income (loss) before income taxes	142		55
Identifiable assets	6,874		7,534
U.S. Public Finance Insurance [Member] | Non Variable Interest Entities [Member]
Segment Reporting Information [Line Items]
Revenues	108	[1]	125	[1]
Net gains (losses) on financial instruments at fair value and foreign exchange	32		10
Losses and loss adjustment	4		14
Operating	14		75
Structured Finance And International Insurance [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	20	[2]	(7)	[2]
Total revenues	70		340
Inter-segment expenses	56	[2]	46	[2]
Total expenses	(66)		238
Income (loss) before income taxes	136		102
Identifiable assets	17,149		19,632
Structured Finance And International Insurance [Member] | Non Variable Interest Entities [Member]
Segment Reporting Information [Line Items]
Revenues	42	[1]	55	[1]
Net change in fair value of insured derivatives	(61)		299
Net gains (losses) on financial instruments at fair value and foreign exchange	22		46
Net investment losses related to other-than-temporary impairments			(40)
Losses and loss adjustment	(198)		83
Other net realized gains (losses)			1
Operating	34		59
Interest	28		33
Structured Finance And International Insurance [Member] | Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Revenues of consolidated VIEs	47		(14)
Expenses of consolidated VIEs	14		17
Advisory Services [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	7	[2]	7	[2]
Total revenues	11		13
Inter-segment expenses	2	[2]	2	[2]
Total expenses	12		17
Income (loss) before income taxes	(1)		(4)
Identifiable assets	34		46
Advisory Services [Member] | Non Variable Interest Entities [Member]
Segment Reporting Information [Line Items]
Revenues	4	[1]	6	[1]
Operating	10		15
Corporate [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	26	[2]	23	[2]
Total revenues	33		29
Inter-segment expenses	3	[2]	4	[2]
Total expenses	79		39
Income (loss) before income taxes	(46)		(10)
Identifiable assets	861		670
Corporate [Member] | Non Variable Interest Entities [Member]
Segment Reporting Information [Line Items]
Revenues	1	[1]	1	[1]
Net gains (losses) on financial instruments at fair value and foreign exchange	6		5
Operating	64		21
Interest	12		14
Wind-Down Operations [Member]
Segment Reporting Information [Line Items]
Net gains (losses) on extinguishment of debt	4
Inter-segment revenues	(2)	[2],[3]	3	[2],[3]
Total revenues	16		(108)
Inter-segment expenses	13	[2],[3]	8	[2],[3]
Total expenses	35		39
Income (loss) before income taxes	(19)		(147)
Identifiable assets	2,484		4,732
Wind-Down Operations [Member] | Non Variable Interest Entities [Member]
Segment Reporting Information [Line Items]
Revenues	9	[1],[4]	19	[1],[4]
Net gains (losses) on financial instruments at fair value and foreign exchange	3		(80)
Net investment losses related to other-than-temporary impairments			(54)
Operating			1
Interest	20		26
Wind-Down Operations [Member] | Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Revenues of consolidated VIEs	2		4
Expenses of consolidated VIEs	2		4
Eliminations [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	(97)	[2]	(62)	[2]
Total revenues	(97)		(62)
Inter-segment expenses	(100)	[2]	(87)	[2]
Total expenses	(100)		(87)
Income (loss) before income taxes	3		25
Identifiable assets	$ (5,810)	[5]	$ (6,488)	[5]

[1] Represents the sum of third-party financial guarantee net premiums earned, net investment income, insurance-related fees and reimbursements, investment management fees and other fees.

[2] Represents intercompany premium income and expense, intercompany asset management fees and expenses, and intercompany interest income and expense pertaining to intercompany receivables and payables.

[3] Represents intercompany asset management fees and expenses plus intercompany interest income and expense pertaining to intercompany debt.

[4] Represents the sum of third-party interest income, investment management services fees and other fees.

[5] Consists of intercompany reinsurance balances, repurchase agreements and loans.