Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	[1]	$ 317		$ 370		$ 480
Net change in fair value of insured derivatives		(25)		(51)		(19)
Net gains (losses) on financial instruments at fair value and foreign exchange		(17)		(24)		84
Net investment losses related to other-than-temporary impairments		(5)		(106)		(5)
Net gains (losses) on extinguishment of debt		3		28		5
Other net realized gains (losses)		0		31		(282)
Revenues of consolidated VIEs		(111)		185		31
Inter-segment revenues	[2]	0		0		0
Total revenues		162		433		294
Losses and loss adjustment		63		683		220
Operating		91		129		177
Interest		206		197		197
Expenses of consolidated VIEs		98		85		39
Inter-segment expenses	[2]	0		0		0
Total expenses		458		1,094		633
Income (loss) before income taxes		(296)		(661)		(339)
Identifiable assets		8,076		9,095		11,137
U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Losses and loss adjustment		0		499		74
Operating Segments [Member] | U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	178		262		335
Net change in fair value of insured derivatives		0		0		0
Net gains (losses) on financial instruments at fair value and foreign exchange		(20)		25		72
Net investment losses related to other-than-temporary impairments		(5)		(106)		(4)
Net gains (losses) on extinguishment of debt		0		0		0
Other net realized gains (losses)		0		(4)		2
Revenues of consolidated VIEs		0		0		0
Inter-segment revenues	[2]	29		23		22
Total revenues		182		200		427
Losses and loss adjustment		91		499		74
Operating		18		36		40
Interest		0		0		0
Expenses of consolidated VIEs		0		0		0
Inter-segment expenses	[2]	44		72		69
Total expenses		153		607		183
Income (loss) before income taxes		29		(407)		244
Identifiable assets		4,203		4,582		5,077
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	31		30		24
Net change in fair value of insured derivatives		0		0		0
Net gains (losses) on financial instruments at fair value and foreign exchange		22		(32)		(14)
Net investment losses related to other-than-temporary impairments		0		0		(1)
Net gains (losses) on extinguishment of debt		3		28		5
Other net realized gains (losses)		(2)		(4)		(5)
Revenues of consolidated VIEs		0		0		0
Inter-segment revenues	[2]	45		60		58
Total revenues		99		82		67
Losses and loss adjustment		0		0		0
Operating		47		59		80
Interest		78		81		91
Expenses of consolidated VIEs		0		0		0
Inter-segment expenses	[2]	20		11		4
Total expenses		145		151		175
Income (loss) before income taxes		(46)		(69)		(108)
Identifiable assets		1,192		1,312		2,479
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	108		78		121
Net change in fair value of insured derivatives		(25)		(51)		(19)
Net gains (losses) on financial instruments at fair value and foreign exchange		(19)		(17)		26
Net investment losses related to other-than-temporary impairments		0		0		0
Net gains (losses) on extinguishment of debt		0		0		0
Other net realized gains (losses)		2		39		(279)
Revenues of consolidated VIEs		(111)		185		31
Inter-segment revenues	[2]	24		44		51
Total revenues		(21)		278		(69)
Losses and loss adjustment		(28)		184		146
Operating		26		34		57
Interest		128		116		106
Expenses of consolidated VIEs		98		85		39
Inter-segment expenses	[2]	33		47		52
Total expenses		257		466		400
Income (loss) before income taxes		(278)		(188)		(469)
Identifiable assets		4,742		5,404		6,486
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	[2]	(98)		(127)		(131)
Total revenues		(98)		(127)		(131)
Inter-segment expenses	[2]	(97)		(130)		(125)
Total expenses		(97)		(130)		(125)
Income (loss) before income taxes		(1)		3		(6)
Identifiable assets		$ (2,061)	[3]	$ (2,203)	[3]	$ (2,905)	[4]

[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 intercompany premium income and expense and intercompany interest income and expense pertaining to intercompany receivables and payables.

[3] Consists primarily of intercompany reinsurance balances and repurchase agreements.

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