Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Mar. 31, 2017
Segment Reporting Information [Line Items]
Revenues	[1]	$ 70	$ 87	$ 147	$ 190
Net change in fair value of insured derivatives		(7)	3	(12)	(50)
Net gains (losses) on financial instruments at fair value and foreign exchange		22	(61)	13	(44)
Net investment losses related to other-than-temporary impairments		(1)	(11)	(2)	(13)
Net gains (losses) on extinguishment of debt		0	0	0	8
Other net realized gains (losses)		0	34	(1)	37
Revenues of consolidated VIEs		(72)	20	(60)	21
Inter-segment revenues	[2]	0	0	0	0
Total revenues		12	72	85	149
Losses and loss adjustment		59	170	131	264
Operating		23	40	47	76
Interest		52	50	103	98
Expenses of consolidated VIEs		24	22	46	41
Inter-segment expenses	[2]	0	0	0	0
Total expenses		158	282	327	479
Income (loss) before income taxes		(146)	(210)	(242)	(330)
Provision (benefit) for income taxes		0	1,019	2	971
Net income (loss)		(146)	(1,229)	(244)	(1,301)
Identifiable assets		8,699		8,699		$ 9,095	$ 9,767
Operating Segments [Member] | U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	41	62	94	130
Net change in fair value of insured derivatives		0	0
Net gains (losses) on financial instruments at fair value and foreign exchange		(8)	14	(14)	18
Net investment losses related to other-than-temporary impairments		(1)	(11)	(2)	(13)
Net gains (losses) on extinguishment of debt		0	0
Other net realized gains (losses)		0	0	0	0
Revenues of consolidated VIEs		0	0
Inter-segment revenues	[2]	6	5	13	10
Total revenues		38	70	91	145
Losses and loss adjustment		59	158	136	169
Operating		6	16	9	26
Interest		0	0
Expenses of consolidated VIEs		0	0
Inter-segment expenses	[2]	9	16	24	31
Total expenses		74	190	169	226
Income (loss) before income taxes		(36)	(120)	(78)	(81)
Provision (benefit) for income taxes		(8)	(43)	(17)	(31)
Net income (loss)		(28)	(77)	(61)	(50)
Identifiable assets		4,412		4,412			5,074
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	7	8	14	16
Net change in fair value of insured derivatives		0	0
Net gains (losses) on financial instruments at fair value and foreign exchange		28	(55)	31	(39)
Net investment losses related to other-than-temporary impairments		0	0		0
Net gains (losses) on extinguishment of debt		0	0	0	8
Other net realized gains (losses)		0	(1)	(2)	(2)
Revenues of consolidated VIEs		0	0
Inter-segment revenues	[2]	11	15	24	31
Total revenues		46	(33)	67	14
Losses and loss adjustment		0	0
Operating		13	14	26	32
Interest		20	22	40	44
Expenses of consolidated VIEs		0	0
Inter-segment expenses	[2]	3	1	9	2
Total expenses		36	37	75	78
Income (loss) before income taxes		10	(70)	(8)	(64)
Provision (benefit) for income taxes		0	1,074	(33)	1,070
Net income (loss)		10	(1,144)	25	(1,134)
Identifiable assets		1,100		1,100			1,233
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	22	17	39	44
Net change in fair value of insured derivatives		(7)	3	(12)	(50)
Net gains (losses) on financial instruments at fair value and foreign exchange		2	(20)	(4)	(23)
Net investment losses related to other-than-temporary impairments		0	0
Net gains (losses) on extinguishment of debt		0	0
Other net realized gains (losses)		0	35	1	39
Revenues of consolidated VIEs		(72)	20	(60)	21
Inter-segment revenues	[2]	6	11	13	20
Total revenues		(49)	66	(23)	51
Losses and loss adjustment		0	12	(5)	95
Operating		4	10	12	18
Interest		32	28	63	54
Expenses of consolidated VIEs		24	22	46	41
Inter-segment expenses	[2]	12	14	18	28
Total expenses		72	86	134	236
Income (loss) before income taxes		(121)	(20)	(157)	(185)
Provision (benefit) for income taxes		0	1,199	(5)	1,142
Net income (loss)		(121)	(1,219)	(152)	(1,327)
Identifiable assets		5,217		5,217			$ 5,313
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	[2]	(23)	(31)	(50)	(61)
Total revenues		(23)	(31)	(50)	(61)
Inter-segment expenses	[2]	(24)	(31)	(51)	(61)
Total expenses		(24)	(31)	(51)	(61)
Income (loss) before income taxes		1	0	1	0
Provision (benefit) for income taxes		8	(1,211)	57	(1,210)
Net income (loss)		(7)	1,211	(56)	1,210
Identifiable assets	[3]	$ (2,030)	$ (1,853)	$ (2,030)	$ (1,853)

[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.