Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Segment Reporting Information [Line Items]
Revenues		$ 480	[1]	$ 530	[1]	$ 616	[2]
Net change in fair value of insured derivatives		(19)		129		459
Net gains (losses) on financial instruments at fair value and foreign exchange		84		63		78
Net investment losses related to other-than-temporary impairments		(5)		(13)		(15)
Net gains (losses) on extinguishment of debt		5		(1)		3
Other net realized gains (losses)		(282)		17		28
Revenues of consolidated VIEs		31		128		101
Inter-segment revenues		0		0		0
Total revenues		294		853		1,270
Losses and loss adjustment		220		123		133
Operating		177		190		239
Interest		197		199		210
Expenses of consolidated VIEs		39		52		47
Inter-segment expenses		0		0		0
Total expenses		633		564		629
Income (loss) before income taxes		(339)		289		641
Provision (benefit) for income taxes		(1)		109		72
Net income (loss)		(338)		180		569
Identifiable assets		11,137		14,836		16,263
U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Losses and loss adjustment		74		5		(10)
Operating Segments [Member] | U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues		335	[1]	389	[1]	373	[2]
Net change in fair value of insured derivatives						1
Net gains (losses) on financial instruments at fair value and foreign exchange		72		14		26
Net investment losses related to other-than-temporary impairments		(4)		(10)		(15)
Other net realized gains (losses)		2		(4)		14
Inter-segment revenues		22	[3]	34	[3]	44	[4]
Total revenues		427		423		443
Losses and loss adjustment		74		5		(10)
Operating		40		38		38
Inter-segment expenses		69	[3]	89	[3]	78	[4]
Total expenses		183		132		106
Income (loss) before income taxes		244		291		337
Provision (benefit) for income taxes		68		100		115
Net income (loss)		176		191		222
Identifiable assets		5,077		5,265		5,887
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Revenues		24	[1]	27	[1]	42	[2]
Net gains (losses) on financial instruments at fair value and foreign exchange		(14)		58		55
Net investment losses related to other-than-temporary impairments		(1)		(3)
Net gains (losses) on extinguishment of debt		5		(1)		3
Other net realized gains (losses)		(5)		21		2
Revenues of consolidated VIEs						0
Inter-segment revenues		58	[3]	67	[3]	46	[4]
Total revenues		67		169		148
Operating		80		79		87
Interest		91		97		101
Inter-segment expenses		4	[3]	7	[3]	15	[4]
Total expenses		175		183		203
Income (loss) before income taxes		(108)		(14)		(55)
Provision (benefit) for income taxes		(15)		7		(173)
Net income (loss)		(93)		(21)		118
Identifiable assets		2,479		2,599		2,838
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues		121	[1]	114	[1]	189	[2]
Net change in fair value of insured derivatives		(19)		129		458
Net gains (losses) on financial instruments at fair value and foreign exchange		26		(9)		0
Other net realized gains (losses)		(279)				12
Revenues of consolidated VIEs		31		128		105
Inter-segment revenues		51	[3]	63	[3]	55	[4]
Total revenues		(69)		425		819
Losses and loss adjustment		146		118		143
Operating		57		73		72
Interest		106		102		109
Expenses of consolidated VIEs		39		52		47
Inter-segment expenses		52	[3]	64	[3]	64	[4]
Total expenses		400		409		435
Income (loss) before income taxes		(469)		16		384
Provision (benefit) for income taxes		(52)		3		134
Net income (loss)		(417)		13		250
Identifiable assets		6,486		9,870		10,073
Operating Segments [Member] | Advisory Services [Member]
Segment Reporting Information [Line Items]
Revenues	[2]					12
Net gains (losses) on financial instruments at fair value and foreign exchange						(3)
Revenues of consolidated VIEs						(8)
Inter-segment revenues	[4]					22
Total revenues						23
Operating						42
Inter-segment expenses	[4]					6
Total expenses						48
Income (loss) before income taxes						(25)
Provision (benefit) for income taxes						(6)
Net income (loss)						(19)
Identifiable assets						793
Operating Segments [Member] | Conduit [Member]
Segment Reporting Information [Line Items]
Revenues of consolidated VIEs						4
Inter-segment revenues	[4]					(1)
Total revenues						3
Expenses of consolidated VIEs						0
Inter-segment expenses	[4]					9
Total expenses						9
Income (loss) before income taxes						(6)
Provision (benefit) for income taxes						0
Net income (loss)						(6)
Identifiable assets						0
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues		(131)	[3]	(164)	[3]	(166)	[4]
Total revenues		(131)		(164)		(166)
Inter-segment expenses		(125)	[3]	(160)	[3]	(172)	[4]
Total expenses		(125)		(160)		(172)
Income (loss) before income taxes		(6)		(4)		6
Provision (benefit) for income taxes		(2)		(1)		2
Net income (loss)		(4)		(3)		4
Identifiable assets		$ (2,905)	[5]	$ (2,898)	[5]	$ (3,328)	[6]

[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, intercompany interest income and expense pertaining to intercompany receivables and payables and intercompany loans.

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

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