Business Segments (Summary Of Company's Segment Results) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Segment Reporting Information [Line Items]
Total revenues	$ 2,435		$ (1,557)		$ 894
Total expenses	837		682		989
Income (loss) before income taxes	1,598		(2,239)		(95)
Identifiable assets	21,724		26,873		32,279
Realized gains (losses) and other settlements on insured derivatives					607
Unrealized gains (losses) on insured derivatives	1,870		(441)		(607)
Net gains (losses) on extinguishment of debt	49		26		60
Losses and loss adjustment	50
Non Variable Interest Entities [Member]
Segment Reporting Information [Line Items]
Revenues	880	[1]	1,038	[1]	1,211	[2]
Realized gains (losses) and other settlements on insured derivatives	(406)		(2,371)		(162)
Unrealized gains (losses) on insured derivatives	1,870		(441)		(607)
Net gains (losses) on financial instruments at fair value and foreign exchange	55		(99)		88
Net investment losses related to other-than-temporary impairments	(105)		(101)		(64)
Net gains (losses) on extinguishment of debt			26		35
Other net realized gains (losses)	7		(1)		29
Losses and loss adjustment	50		(80)		232
Operating	431		371		349
Interest	284		300		325
Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Identifiable assets	8,300		10,900
Net gains (losses) on financial instruments at fair value and foreign exchange	18		59		342
Net gains (losses) on extinguishment of debt	49				25
Other net realized gains (losses)			263		(76)
Revenues of consolidated VIEs	134		392		364
Interest	55		62		59
Expenses of consolidated VIEs	72		91		83
U.S. Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	168	[3]	79	[4]	93	[5]
Total revenues	838		745		750
Inter-segment expenses	124	[3]	114	[6]	104	[5]
Total expenses	269		170		220
Income (loss) before income taxes	569		575		530
Identifiable assets	6,887		7,848		8,436
U.S. Public Finance Insurance [Member] | Non Variable Interest Entities [Member]
Segment Reporting Information [Line Items]
Revenues	548	[1]	599	[1]	601	[2]
Realized gains (losses) and other settlements on insured derivatives	1		2		1
Net gains (losses) on financial instruments at fair value and foreign exchange	121		96		55
Other net realized gains (losses)			(31)
Losses and loss adjustment	21		4		73
Operating	124		52		43
Structured Finance And International Insurance [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	45	[3]	91	[4]	113	[5]
Total revenues	1,871		(2,046)		211
Inter-segment expenses	208	[3]	124	[6]	135	[5]
Total expenses	575		409		642
Income (loss) before income taxes	1,296		(2,455)		(431)
Identifiable assets	17,264		19,985		23,980
Structured Finance And International Insurance [Member] | Non Variable Interest Entities [Member]
Segment Reporting Information [Line Items]
Revenues	243	[1]	319	[1]	463	[2]
Realized gains (losses) and other settlements on insured derivatives	(407)		(2,373)		(163)
Unrealized gains (losses) on insured derivatives	1,870		(441)		(607)
Net gains (losses) on financial instruments at fair value and foreign exchange	93		58		135
Net investment losses related to other-than-temporary impairments	(45)		(62)		(5)
Other net realized gains (losses)	1		1		29
Losses and loss adjustment	29		(84)		159
Operating	147		166		149
Interest	132		133		135
Structured Finance And International Insurance [Member] | Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Revenues of consolidated VIEs	71		361		246
Expenses of consolidated VIEs	59		70		64
Advisory Services [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	34	[3]	41	[4]	38	[5]
Total revenues	54		67		70
Inter-segment expenses	13	[3]	6	[6]	7	[5]
Total expenses	59		64		71
Income (loss) before income taxes	(5)		3		(1)
Identifiable assets	46		53		54
Advisory Services [Member] | Non Variable Interest Entities [Member]
Segment Reporting Information [Line Items]
Revenues	21	[1]	26	[1]	30	[2]
Net gains (losses) on financial instruments at fair value and foreign exchange	(1)				2
Operating	46		58		64
Corporate [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	176	[3]	152	[4]	94	[5]
Total revenues	208		196		70
Inter-segment expenses	14	[3]	22	[6]	14	[5]
Total expenses	180		172		168
Income (loss) before income taxes	28		24		(98)
Identifiable assets	828		829		653
Corporate [Member] | Non Variable Interest Entities [Member]
Segment Reporting Information [Line Items]
Revenues	15	[1]	4	[1]	4	[2]
Net gains (losses) on financial instruments at fair value and foreign exchange	18		23		(28)
Net investment losses related to other-than-temporary impairments	(4)		(8)
Net gains (losses) on extinguishment of debt	(2)
Other net realized gains (losses)	5		25
Operating	109		92		89
Interest	57		58		65
Wind-Down Operations [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	(14)	[3],[7]	(23)	[4],[7]	(18)	[5],[7]
Total revenues	(127)		(181)		91
Inter-segment expenses	116	[3],[7]	95	[6],[7]	60	[5],[7]
Total expenses	229		228		208
Income (loss) before income taxes	(356)		(409)		(117)
Identifiable assets	2,539		5,203		6,432
Wind-Down Operations [Member] | Non Variable Interest Entities [Member]
Segment Reporting Information [Line Items]
Revenues	53	[1],[8]	90	[1],[8]	113	[2],[8]
Net gains (losses) on financial instruments at fair value and foreign exchange	(176)		(276)		(76)
Net investment losses related to other-than-temporary impairments	(56)		(31)		(59)
Net gains (losses) on extinguishment of debt	2		24		35
Other net realized gains (losses)	1		4
Operating	5		3		4
Interest	95		109		125
Wind-Down Operations [Member] | Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Revenues of consolidated VIEs	63		31		96
Expenses of consolidated VIEs	13		21		19
Eliminations [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	(409)	[3]	(340)	[4]	(320)	[5]
Total revenues	(409)		(338)		(298)
Inter-segment expenses	(475)	[3]	(361)	[6]	(320)	[5]
Total expenses	(475)		(361)		(320)
Income (loss) before income taxes	66		23		22
Identifiable assets	(5,840)	[6]	(7,045)	[9]	(7,276)	[10]
Eliminations [Member] | Non Variable Interest Entities [Member]
Segment Reporting Information [Line Items]
Net gains (losses) on extinguishment of debt			2
Eliminations [Member] | Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Revenues of consolidated VIEs					$ 22

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

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

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

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

[6] Consists of intercompany reinsurance balances, repurchase agreements, deferred taxes, and loans.

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

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

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

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