Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Revenues	[1]	$ 50	$ 34	$ 114	$ 116
Net gains (losses) on financial instruments at fair value and foreign exchange		11	13	42	(26)
Net gains (losses) on extinguishment of debt		16		30
Revenues of consolidated VIEs		(5)	24	(24)	89
Inter-segment revenues	[2]	0	0	0	0
Total revenues		72	71	162	179
Losses and loss adjustment		125	48	232	427
Operating		23	23	75	68
Interest		40	43	122	135
Expenses of consolidated VIEs		7	15	23	46
Inter-segment expenses	[2]	0	0	0	0
Total expenses		195	129	452	676
Income (loss) before income taxes		(123)	(58)	(290)	(497)
Identifiable assets		4,816		4,816		$ 5,751
Operating Segments [Member] | US Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	20	21	64	78
Net gains (losses) on financial instruments at fair value and foreign exchange		1	23	1	30
Net gains (losses) on extinguishment of debt		0		0
Revenues of consolidated VIEs		0	0	0	0
Inter-segment revenues	[2]	8	7	22	21
Total revenues		29	51	87	129
Losses and loss adjustment		68	14	135	134
Operating		6	3	14	8
Interest		0	0	0	0
Expenses of consolidated VIEs		0	0	0	0
Inter-segment expenses	[2]	11	9	36	34
Total expenses		85	26	185	176
Income (loss) before income taxes		(56)	25	(98)	(47)
Identifiable assets		3,448		3,448
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	6	4	13	15
Net gains (losses) on financial instruments at fair value and foreign exchange		13	(6)	50	(62)
Net gains (losses) on extinguishment of debt		16		30
Revenues of consolidated VIEs		0	0	0	0
Inter-segment revenues	[2]	14	16	50	51
Total revenues		49	14	143	4
Losses and loss adjustment		0	0	0	0
Operating		16	16	54	50
Interest		14	16	42	48
Expenses of consolidated VIEs		0	0	0	0
Inter-segment expenses	[2]	4	6	14	17
Total expenses		34	38	110	115
Income (loss) before income taxes		15	(24)	33	(111)
Identifiable assets		826		826
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	24	9	37	23
Net gains (losses) on financial instruments at fair value and foreign exchange		(3)	(4)	(9)	6
Net gains (losses) on extinguishment of debt		0		0
Revenues of consolidated VIEs		(5)	24	(24)	89
Inter-segment revenues	[2]	2	(1)	10	7
Total revenues		18	28	14	125
Losses and loss adjustment		57	34	97	293
Operating		1	4	7	10
Interest		26	27	80	87
Expenses of consolidated VIEs		7	15	23	46
Inter-segment expenses	[2]	9	7	30	28
Total expenses		100	87	237	464
Income (loss) before income taxes		(82)	(59)	(223)	(339)
Identifiable assets		3,034		3,034
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]	(24)	(22)	(82)	(79)
Total revenues		(24)	(22)	(82)	(79)
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]	(24)	(22)	(80)	(79)
Total expenses		(24)	(22)	(80)	(79)
Income (loss) before income taxes		0	$ 0	(2)	$ 0
Identifiable assets	[3]	$ (2,492)		$ (2,492)

[1] Represents the sum of third-party financial guarantee net premiums earned, net investment income, insurance-related fees and reimbursements and other fees.

[2] Primarily represents intercompany service charges and intercompany net investment income and expense.

[3] Consists principally of intercompany reinsurance balances.