Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Revenues	[1]	$ 34	$ 47	$ 116	$ 155
Net change in fair value of insured derivatives					13
Net gains (losses) on financial instruments at fair value and foreign exchange		13	44	(26)	40
Net investment losses related to other-than-temporary impairments					(37)
Net gains (losses) on extinguishment of debt			(1)		(1)
Other net realized gains (losses)			0		2
Revenues of consolidated VIEs		24	82	89	80
Inter-segment revenues	[2]	0	0	0	0
Total revenues		71	172	179	252
Losses and loss adjustment		48	(13)	427	89
Operating		23	26	68	77
Interest		43	50	135	154
Expenses of consolidated VIEs		15	20	46	69
Inter-segment expenses	[2]	0	0	0	0
Total expenses		129	83	676	389
Income (loss) before income taxes		(58)	89	(497)	(137)
Identifiable assets		6,551		6,551		$ 7,284
Operating Segments [Member] | U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	21	30	78	106
Net change in fair value of insured derivatives					0
Net gains (losses) on financial instruments at fair value and foreign exchange		23	78	30	135
Net investment losses related to other-than-temporary impairments					(37)
Net gains (losses) on extinguishment of debt			0		0
Other net realized gains (losses)			0		1
Revenues of consolidated VIEs		0	32	0	38
Inter-segment revenues	[2]	7	7	21	22
Total revenues		51	147	129	265
Losses and loss adjustment		14	(90)	134	(34)
Operating		3	4	8	8
Interest		0	0	0	0
Expenses of consolidated VIEs		0	0	0	0
Inter-segment expenses	[2]	9	14	34	41
Total expenses		26	(72)	176	15
Income (loss) before income taxes		25	219	(47)	250
Identifiable assets		3,777		3,777
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	4	6	15	21
Net change in fair value of insured derivatives					0
Net gains (losses) on financial instruments at fair value and foreign exchange		(6)	(20)	(62)	(71)
Net investment losses related to other-than-temporary impairments					0
Net gains (losses) on extinguishment of debt			(1)		(1)
Other net realized gains (losses)			(1)		(2)
Revenues of consolidated VIEs		0	0	0	0
Inter-segment revenues	[2]	16	15	51	48
Total revenues		14	(1)	4	(5)
Losses and loss adjustment		0	0	0	0
Operating		16	15	50	53
Interest		16	17	48	56
Expenses of consolidated VIEs		0	0	0	0
Inter-segment expenses	[2]	6	6	17	17
Total expenses		38	38	115	126
Income (loss) before income taxes		(24)	(39)	(111)	(131)
Identifiable assets		1,031		1,031
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	9	11	23	28
Net change in fair value of insured derivatives					13
Net gains (losses) on financial instruments at fair value and foreign exchange		(4)	(14)	6	(24)
Net investment losses related to other-than-temporary impairments					0
Net gains (losses) on extinguishment of debt			0		0
Other net realized gains (losses)			1		3
Revenues of consolidated VIEs		24	50	89	41
Inter-segment revenues	[2]	(1)	6	7	15
Total revenues		28	54	125	76
Losses and loss adjustment		34	77	293	123
Operating		4	7	10	16
Interest		27	33	87	98
Expenses of consolidated VIEs		15	20	46	69
Inter-segment expenses	[2]	7	8	28	26
Total expenses		87	145	464	332
Income (loss) before income taxes		(59)	(91)	(339)	(256)
Identifiable assets		4,309		4,309
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	0	0	0	0
Net change in fair value of insured derivatives					0
Net gains (losses) on financial instruments at fair value and foreign exchange		0	0	0	0
Net investment losses related to other-than-temporary impairments					0
Net gains (losses) on extinguishment of debt			0		0
Other net realized gains (losses)			0		0
Revenues of consolidated VIEs		0	0	0	1
Inter-segment revenues	[2]	(22)	(28)	(79)	(85)
Total revenues		(22)	(28)	(79)	(84)
Losses and loss adjustment		0	0	0	0
Operating		0	0	0	0
Interest		0	0	0	0
Expenses of consolidated VIEs		0	0	0	0
Inter-segment expenses	[2]	(22)	(28)	(79)	(84)
Total expenses		(22)	(28)	(79)	(84)
Income (loss) before income taxes		0	$ 0	0	$ 0
Identifiable assets	[3]	$ (2,566)		$ (2,566)

[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.