Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net change in fair value of insured derivatives				$ (51)
Net investment losses related to other-than-temporary impairments				(106)
Net gains (losses) on extinguishment of debt		$ (1)
Total revenues		280	$ 162	433
Total expenses		637	458	1,094
Income (loss) before income taxes		(357)	(296)	(661)
Identifiable assets		7,284	8,107
Non Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	200	317	370
Net change in fair value of insured derivatives		15	(25)	(51)
Net gains (losses) on financial instruments at fair value and foreign exchange		52	(17)	(24)
Net investment losses related to other-than-temporary impairments		(67)	(5)	(106)
Net gains (losses) on extinguishment of debt		(1)	3	28
Other net realized gains (losses)		4	0	31
Losses and loss adjustment		242	63	683
Operating		103	91	129
Interest		201	206	197
Variable Interest Entity Primary Beneficiary [Member]
Segment Reporting Information [Line Items]
Net gains (losses) on financial instruments at fair value and foreign exchange		105	25	130
Other net realized gains (losses)		(62)	(171)	28
Revenues of consolidated VIEs		77	(111)	185
Interest		82	87	75
Expenses of consolidated VIEs		91	98	85
Identifiable assets		1,600	1,700
Operating Segments [Member] | U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	[2]	28	29	23
Total revenues		262	182	200
Inter-segment expenses	[2]	52	44	72
Total expenses		118	153	607
Income (loss) before income taxes		144	29	(407)
Identifiable assets		4,019	4,203
Operating Segments [Member] | U S Public Finance Insurance [Member] | Non Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	139	178	262
Net gains (losses) on financial instruments at fair value and foreign exchange		139	(20)	25
Net investment losses related to other-than-temporary impairments		(67)	(5)	(106)
Other net realized gains (losses)		2	0	(4)
Losses and loss adjustment		53	91	499
Operating		13	18	36
Operating Segments [Member] | U S Public Finance Insurance [Member] | Variable Interest Entity Primary Beneficiary [Member]
Segment Reporting Information [Line Items]
Revenues of consolidated VIEs		21	0	0
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	[2]	62	45	60
Total revenues		33	99	82
Inter-segment expenses	[2]	23	20	11
Total expenses		165	145	151
Income (loss) before income taxes		(132)	(46)	(69)
Identifiable assets		1,041	1,192
Operating Segments [Member] | Corporate Operations [Member] | Non Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	27	31	30
Net gains (losses) on financial instruments at fair value and foreign exchange		(54)	22	(32)
Net gains (losses) on extinguishment of debt		(1)	3	28
Other net realized gains (losses)			(2)	(4)
Operating		69	47	59
Interest		73	78	81
Operating Segments [Member] | Corporate Operations [Member] | Variable Interest Entity Primary Beneficiary [Member]
Segment Reporting Information [Line Items]
Revenues of consolidated VIEs		1	0	0
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	[2]	21	24	44
Total revenues		96	(21)	278
Inter-segment expenses	[2]	36	33	47
Total expenses		465	257	466
Income (loss) before income taxes		(369)	(278)	(188)
Identifiable assets		4,504	4,773
Operating Segments [Member] | International And Structured Finance Insurance [Member] | Non Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	34	108	78
Net change in fair value of insured derivatives		15	(25)	(51)
Net gains (losses) on financial instruments at fair value and foreign exchange		(33)	(19)	(17)
Other net realized gains (losses)		4	2	39
Losses and loss adjustment		189	(28)	184
Operating		21	26	34
Interest		128	128	116
Operating Segments [Member] | International And Structured Finance Insurance [Member] | Variable Interest Entity Primary Beneficiary [Member]
Segment Reporting Information [Line Items]
Revenues of consolidated VIEs		55	(111)	185
Expenses of consolidated VIEs		91	98	85
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	[2]	(111)	(98)	(127)
Total revenues		(111)	(98)	(127)
Inter-segment expenses	[2]	(111)	(97)	(130)
Total expenses		(111)	(97)	(130)
Income (loss) before income taxes		0	(1)	$ 3
Identifiable assets	[3]	$ (2,280)	$ (2,061)

[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.