Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	[1]	$ 110	$ 87	$ 257	$ 277
Net change in fair value of insured derivatives		(1)	(1)	(13)	(51)
Net gains (losses) on financial instruments at fair value and foreign exchange		5	(11)	18	(55)
Net investment losses related to other-than-temporary impairments		(1)	(71)	(3)	(84)
Net gains (losses) on extinguishment of debt		3	1	3	9
Other net realized gains (losses)		1	(1)	0	36
Revenues of consolidated VIEs		(12)	29	(72)	50
Inter-segment revenues	[2]	0	0	0	0
Total revenues		105	33	190	182
Losses and loss adjustment		46	205	177	469
Operating		27	29	74	105
Interest		52	50	155	148
Expenses of consolidated VIEs		25	22	71	63
Inter-segment expenses	[2]	0	0	0	0
Total expenses		150	306	477	785
Income (loss) before income taxes		(45)	(273)	(287)	(603)
Provision (benefit) for income taxes		0	(6)	2	965
Net income (loss)		(45)	(267)	(289)	(1,568)
Identifiable assets		8,361	9,544	8,361	9,544	$ 9,095
Operating Segments [Member] | U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	45	70	139	200
Net change in fair value of insured derivatives		0	0	0	0
Net gains (losses) on financial instruments at fair value and foreign exchange		1	2	(13)	20
Net investment losses related to other-than-temporary impairments		(1)	(71)	(3)	(84)
Net gains (losses) on extinguishment of debt		0	0	0	0
Other net realized gains (losses)		0	(1)	0	(1)
Revenues of consolidated VIEs		0	0	0	0
Inter-segment revenues	[2]	7	4	20	14
Total revenues		52	4	143	149
Losses and loss adjustment		48	141	184	310
Operating		5	8	14	34
Interest		0	0	0	0
Expenses of consolidated VIEs		0	0	0	0
Inter-segment expenses	[2]	10	16	34	47
Total expenses		63	165	232	391
Income (loss) before income taxes		(11)	(161)	(89)	(242)
Provision (benefit) for income taxes		(3)	(55)	(20)	(86)
Net income (loss)		(8)	(106)	(69)	(156)
Identifiable assets		4,453	5,051	4,453	5,051
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	7	7	20	23
Net change in fair value of insured derivatives		0	0	0	0
Net gains (losses) on financial instruments at fair value and foreign exchange		17	(15)	48	(54)
Net investment losses related to other-than-temporary impairments		0	0	0	0
Net gains (losses) on extinguishment of debt		3	1	3	9
Other net realized gains (losses)		0	(1)	(2)	(3)
Revenues of consolidated VIEs		0	0	0	0
Inter-segment revenues	[2]	11	15	36	46
Total revenues		38	7	105	21
Losses and loss adjustment		0	0	0	0
Operating		13	14	39	46
Interest		20	22	60	66
Expenses of consolidated VIEs		0	0	0	0
Inter-segment expenses	[2]	5	0	14	2
Total expenses		38	36	113	114
Income (loss) before income taxes		0	(29)	(8)	(93)
Provision (benefit) for income taxes		2	(1)	(31)	1,069
Net income (loss)		(2)	(28)	23	(1,162)
Identifiable assets		1,062	1,205	1,062	1,205
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	58	10	98	54
Net change in fair value of insured derivatives		(1)	(1)	(13)	(51)
Net gains (losses) on financial instruments at fair value and foreign exchange		(13)	2	(17)	(21)
Net investment losses related to other-than-temporary impairments		0	0	0	0
Net gains (losses) on extinguishment of debt		0	0	0	0
Other net realized gains (losses)		1	1	2	40
Revenues of consolidated VIEs		(12)	29	(72)	50
Inter-segment revenues	[2]	6	11	18	31
Total revenues		39	52	16	103
Losses and loss adjustment		(2)	64	(7)	159
Operating		9	7	21	25
Interest		32	28	95	82
Expenses of consolidated VIEs		25	22	71	63
Inter-segment expenses	[2]	9	14	27	42
Total expenses		73	135	207	371
Income (loss) before income taxes		(34)	(83)	(191)	(268)
Provision (benefit) for income taxes		0	1	(5)	1,143
Net income (loss)		(34)	(84)	(186)	(1,411)
Identifiable assets		5,026	5,320	5,026	5,320
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	[2]	(24)	(30)	(74)	(91)
Total revenues		(24)	(30)	(74)	(91)
Inter-segment expenses	[2]	(24)	(30)	(75)	(91)
Total expenses		(24)	(30)	(75)	(91)
Income (loss) before income taxes		0	0	1	0
Provision (benefit) for income taxes		1	49	58	(1,161)
Net income (loss)		(1)	(49)	(57)	1,161
Identifiable assets	[3]	$ (2,180)	$ (2,032)	$ (2,180)	$ (2,032)

[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.