Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2015		Jun. 30, 2014		Jun. 30, 2015		Jun. 30, 2014		Dec. 31, 2014
Segment Reporting Information [Line Items]
Revenues		$ 130	[1]	$ 135	[2]	$ 269	[1]	$ 277	[2]
Net change in fair value of insured derivatives		60		(47)		88		422
Net gains (losses) on financial instruments at fair value and foreign exchange		45		61		75		6
Net investment losses related to other-than-temporary impairments		(7)				(7)
Net gains (losses) on extinguishment of debt		(1)		2		(1)		3
Other net realized gains (losses)		(1)				19		1
Revenues of consolidated VIEs		19		36		21		55
Inter-segment revenues		0	[3]	0	[4]	0	[3]	0	[4]
Total revenues		245		187		464		764
Losses and loss adjustment		46		12		40		62
Operating		45		57		93		113
Interest		50		52		100		106
Expenses of consolidated VIEs		12		11		26		24
Inter-segment expenses		0	[3]	0	[4]	0	[3]	0	[4]
Total expenses		153		132		259		305
Income (loss) before income taxes		92		55		205		459
Provision (benefit) for income taxes		28		(65)		72		83
Net income (loss)		64		120		133		376
Identifiable assets		15,510		16,024		15,510		16,024		$ 16,284
U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Losses and loss adjustment		8		17		2		3
Operating Segments [Member] | U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues		93	[1]	84	[2]	198	[1]	172	[2]
Net change in fair value of insured derivatives								1
Net gains (losses) on financial instruments at fair value and foreign exchange				15		3		19
Net investment losses related to other-than-temporary impairments		(6)				(6)
Other net realized gains (losses)						(4)
Inter-segment revenues		8	[3]	11	[4]	18	[3]	23	[4]
Total revenues		95		110		209		215
Losses and loss adjustment		8		17		2		3
Operating		8		11		18		20
Inter-segment expenses		22	[3]	20	[4]	46	[3]	38	[4]
Total expenses		38		48		66		61
Income (loss) before income taxes		57		62		143		154
Provision (benefit) for income taxes		20		21		49		52
Net income (loss)		37		41		94		102
Identifiable assets		5,412		6,194		5,412		6,194
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues		29	[1]	43	[2]	58	[1]	75	[2]
Net change in fair value of insured derivatives		60		(47)		88		421
Net gains (losses) on financial instruments at fair value and foreign exchange		(3)		5		(8)		2
Revenues of consolidated VIEs		19		31		21		51
Inter-segment revenues		15	[3]	12	[4]	33	[3]	25	[4]
Total revenues		120		44		192		574
Losses and loss adjustment		38		(5)		38		59
Operating		18		15		37		32
Interest		26		26		51		54
Expenses of consolidated VIEs		12		11		26		24
Inter-segment expenses		15	[3]	17	[4]	33	[3]	35	[4]
Total expenses		109		64		185		204
Income (loss) before income taxes		11		(20)		7		370
Provision (benefit) for income taxes		3		(8)		1		129
Net income (loss)		8		(12)		6		241
Identifiable assets		10,393		10,702		10,393		10,702
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Revenues		8	[1]	4	[2]	13	[1]	23	[2]
Net gains (losses) on financial instruments at fair value and foreign exchange		48		40		80		(12)
Net investment losses related to other-than-temporary impairments		(1)				(1)
Net gains (losses) on extinguishment of debt		(1)		2		(1)		3
Other net realized gains (losses)		(1)				23		1
Revenues of consolidated VIEs				5
Inter-segment revenues		21	[3]	19	[4]	37	[3]	26	[4]
Total revenues		74		70		151		41
Operating		19		20		38		39
Interest		24		26		49		52
Inter-segment expenses		1	[3]	3	[4]	2	[3]	7	[4]
Total expenses		44		49		89		98
Income (loss) before income taxes		30		21		62		(57)
Provision (benefit) for income taxes		7		(76)		25		(97)
Net income (loss)		23		97		37		40
Identifiable assets		2,707		2,702		2,707		2,702
Operating Segments [Member] | Advisory Services [Member]
Segment Reporting Information [Line Items]
Revenues	[2]			4				7
Net gains (losses) on financial instruments at fair value and foreign exchange				1				(3)
Inter-segment revenues	[4]			6				12
Total revenues				11				16
Operating				11				22
Inter-segment expenses	[4]			2				3
Total expenses				13				25
Income (loss) before income taxes				(2)				(9)
Provision (benefit) for income taxes				0				(1)
Net income (loss)				(2)				(8)
Identifiable assets				38				38
Operating Segments [Member] | Conduit [Member]
Segment Reporting Information [Line Items]
Revenues of consolidated VIEs								4
Inter-segment revenues	[4]							(1)
Total revenues				0				3
Inter-segment expenses	[4]			9				9
Total expenses				9				9
Income (loss) before income taxes				(9)				(6)
Provision (benefit) for income taxes				(3)				(2)
Net income (loss)				(6)				(4)
Identifiable assets				0				0
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues		(44)	[3]	(48)	[4]	(88)	[3]	(85)	[4]
Total revenues		(44)		(48)		(88)		(85)
Inter-segment expenses		(38)	[3]	(51)	[4]	(81)	[3]	(92)	[4]
Total expenses		(38)		(51)		(81)		(92)
Income (loss) before income taxes		(6)		3		(7)		7
Provision (benefit) for income taxes		(2)		1		(3)		2
Net income (loss)		(4)		2		(4)		5
Identifiable assets	[5]	$ (3,002)		$ (3,612)		$ (3,002)		$ (3,612)

[1] Represents the sum of third-party financial guarantee net premiums earned, net investment income, insurance-related fees and reimbursements 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 and intercompany interest income and expense pertaining to intercompany receivables and payables.

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