Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	[1]	$ 151		$ 200		$ 317
Net change in fair value of insured derivatives		6		15		(25)
Net gains (losses) on financial instruments at fair value and foreign exchange		(38)		52		(17)
Net investment losses related to other-than-temporary impairments				(67)		(5)
Net gains (losses) on extinguishment of debt				(1)		3
Other net realized gains (losses)		0		4		0
Revenues of consolidated VIEs		163		77		(111)
Inter-segment revenues	[2]	0		0		0
Total revenues		282		280		162
Losses and loss adjustment		530		242		63
Operating		97		103		91
Interest		178		201		206
Expenses of consolidated VIEs		55		91		98
Inter-segment expenses	[2]	0		0		0
Total expenses		860		637		458
Income (loss) before income taxes		(578)		(357)		(296)
Identifiable assets		5,751		7,284
Operating Segments [Member] | U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	102		139		178
Net change in fair value of insured derivatives		0		0		0
Net gains (losses) on financial instruments at fair value and foreign exchange		39		139		(20)
Net investment losses related to other-than-temporary impairments				(67)		(5)
Net gains (losses) on extinguishment of debt				0		0
Other net realized gains (losses)		(1)		2		0
Revenues of consolidated VIEs		0		21		0
Inter-segment revenues	[2]	28		28		29
Total revenues		168		262		182
Losses and loss adjustment		163		53		91
Operating		14		13		18
Interest		0		0		0
Expenses of consolidated VIEs		0		0		0
Inter-segment expenses	[2]	45		52		44
Total expenses		222		118		153
Income (loss) before income taxes		(54)		144		29
Identifiable assets		3,644		4,019
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	20		27		31
Net change in fair value of insured derivatives		0		0		0
Net gains (losses) on financial instruments at fair value and foreign exchange		(63)		(54)		22
Net investment losses related to other-than-temporary impairments				0		0
Net gains (losses) on extinguishment of debt				(1)		3
Other net realized gains (losses)		0		(2)		(2)
Revenues of consolidated VIEs		0		1		0
Inter-segment revenues	[2]	66		62		45
Total revenues		23		33		99
Losses and loss adjustment		0		0		0
Operating		69		69		47
Interest		65		73		78
Expenses of consolidated VIEs		0		0		0
Inter-segment expenses	[2]	22		23		20
Total expenses		156		165		145
Income (loss) before income taxes		(133)		(132)		(46)
Identifiable assets		954		1,041
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	29		34		108
Net change in fair value of insured derivatives		6		15		(25)
Net gains (losses) on financial instruments at fair value and foreign exchange		(14)		(33)		(19)
Net investment losses related to other-than-temporary impairments				0		0
Net gains (losses) on extinguishment of debt				0		0
Other net realized gains (losses)		1		4		2
Revenues of consolidated VIEs		163		55		(111)
Inter-segment revenues	[2]	12		21		24
Total revenues		197		96		(21)
Losses and loss adjustment		367		189		(28)
Operating		14		21		26
Interest		113		128		128
Expenses of consolidated VIEs		55		91		98
Inter-segment expenses	[2]	39		36		33
Total expenses		588		465		257
Income (loss) before income taxes		(391)		(369)		(278)
Identifiable assets		3,671		4,504
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	0		0		0
Net change in fair value of insured derivatives		0		0		0
Net gains (losses) on financial instruments at fair value and foreign exchange		0		0		0
Net investment losses related to other-than-temporary impairments				0		0
Net gains (losses) on extinguishment of debt				0		0
Other net realized gains (losses)		0		0		0
Revenues of consolidated VIEs		0		0		0
Inter-segment revenues	[2]	(106)		(111)		(98)
Total revenues		(106)		(111)		(98)
Losses and loss adjustment		0		0		0
Operating		0		0		0
Interest		0		0		0
Expenses of consolidated VIEs		0		0		0
Inter-segment expenses	[2]	(106)		(111)		(97)
Total expenses		(106)		(111)		(97)
Income (loss) before income taxes		0		0		$ (1)
Identifiable assets		$ (2,518)	[3]	$ (2,280)	[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 principally of intercompany reinsurance balances.

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