Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Revenues	[1]	$ 29	$ 39	$ 64	$ 82
Net gains (losses) on financial instruments at fair value and foreign exchange		(20)	24	31	(39)
Net gains (losses) on extinguishment of debt		14		14
Revenues of consolidated VIEs		(5)	51	(19)	65
Inter-segment revenues	[2]	0	0	0	0
Total revenues		18	114	90	108
Losses and loss adjustment		9	136	107	379
Operating		24	25	52	45
Interest		41	45	82	92
Expenses of consolidated VIEs		5	14	16	31
Inter-segment expenses	[2]	0	0	0	0
Total expenses		79	220	257	547
Income (loss) before income taxes		(61)	(106)	(167)	(439)
Identifiable assets		5,252		5,252		$ 5,751
Operating Segments [Member] | US Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	21	26	44	57
Net gains (losses) on financial instruments at fair value and foreign exchange		3	25	0	7
Net gains (losses) on extinguishment of debt		0		0
Revenues of consolidated VIEs		0	0	0	0
Inter-segment revenues	[2]	6	8	14	14
Total revenues		30	59	58	78
Losses and loss adjustment		(42)	72	67	120
Operating		4	3	8	5
Interest		0	0	0	0
Expenses of consolidated VIEs		0	0	0	0
Inter-segment expenses	[2]	11	11	25	25
Total expenses		(27)	86	100	150
Income (loss) before income taxes		57	(27)	(42)	(72)
Identifiable assets		3,593		3,593
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	2	5	7	11
Net gains (losses) on financial instruments at fair value and foreign exchange		(18)	0	37	(56)
Net gains (losses) on extinguishment of debt		14		14
Revenues of consolidated VIEs		0	0	0	0
Inter-segment revenues	[2]	18	17	36	35
Total revenues		16	22	94	(10)
Losses and loss adjustment		0	0	0	0
Operating		17	19	38	34
Interest		14	16	28	32
Expenses of consolidated VIEs		0	0	0	0
Inter-segment expenses	[2]	4	6	10	11
Total expenses		35	41	76	77
Income (loss) before income taxes		(19)	(19)	18	(87)
Identifiable assets		852		852
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	6	8	13	14
Net gains (losses) on financial instruments at fair value and foreign exchange		(5)	(1)	(6)	10
Net gains (losses) on extinguishment of debt		0		0
Revenues of consolidated VIEs		(5)	51	(19)	65
Inter-segment revenues	[2]	4	3	8	8
Total revenues		0	61	(4)	97
Losses and loss adjustment		51	64	40	259
Operating		3	3	6	6
Interest		27	29	54	60
Expenses of consolidated VIEs		5	14	16	31
Inter-segment expenses	[2]	11	11	21	21
Total expenses		97	121	137	377
Income (loss) before income taxes		(97)	(60)	(141)	(280)
Identifiable assets		3,346		3,346
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	0	0	0	0
Net gains (losses) on financial instruments at fair value and foreign exchange		0	0	0	0
Net gains (losses) on extinguishment of debt		0		0
Revenues of consolidated VIEs		0	0	0	0
Inter-segment revenues	[2]	(28)	(28)	(58)	(57)
Total revenues		(28)	(28)	(58)	(57)
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]	(26)	(28)	(56)	(57)
Total expenses		(26)	(28)	(56)	(57)
Income (loss) before income taxes		(2)	$ 0	(2)	$ 0
Identifiable assets	[3]	$ (2,539)		$ (2,539)

[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.