Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	[1]	$ 103	$ 115
Net change in fair value of insured derivatives		(53)	(28)
Net gains (losses) on financial instruments at fair value and foreign exchange		17	(69)
Net investment losses related to other-than-temporary impairments		(2)	(1)
Net gains (losses) on extinguishment of debt		8	2
Other net realized gains (losses)		3	(1)
Revenues of consolidated VIEs		1	14
Inter-segment revenues	[2]	0	0
Total revenues		77	32
Losses and loss adjustment		94	22
Operating		36	45
Interest		48	50
Expenses of consolidated VIEs		19	16
Inter-segment expenses	[2]	0	0
Total expenses		197	133
Income (loss) before income taxes		(120)	(101)
Provision (benefit) for income taxes		(48)	(23)
Net income (loss)		(72)	(78)
Identifiable assets		11,017	12,751	$ 11,137
U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Losses and loss adjustment		11	9
Operating Segments [Member] | U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	68	84
Net gains (losses) on financial instruments at fair value and foreign exchange		4	9
Net investment losses related to other-than-temporary impairments		(2)
Inter-segment revenues	[2]	5	6
Total revenues		75	99
Losses and loss adjustment		11	9
Operating		10	9
Inter-segment expenses	[2]	15	18
Total expenses		36	36
Income (loss) before income taxes		39	63
Provision (benefit) for income taxes		12	22
Net income (loss)		27	41
Identifiable assets		5,128	5,268
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	8	5
Net gains (losses) on financial instruments at fair value and foreign exchange		16	(84)
Net investment losses related to other-than-temporary impairments			(1)
Net gains (losses) on extinguishment of debt		8	2
Other net realized gains (losses)		(1)	(1)
Inter-segment revenues	[2]	16	16
Total revenues		47	(63)
Operating		18	22
Interest		22	23
Inter-segment expenses	[2]	1
Total expenses		41	45
Income (loss) before income taxes		6	(108)
Provision (benefit) for income taxes		(4)	(24)
Net income (loss)		10	(84)
Identifiable assets		2,335	2,478
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	27	26
Net change in fair value of insured derivatives		(53)	(28)
Net gains (losses) on financial instruments at fair value and foreign exchange		(3)	6
Other net realized gains (losses)		4
Revenues of consolidated VIEs		1	14
Inter-segment revenues	[2]	9	12
Total revenues		(15)	30
Losses and loss adjustment		83	13
Operating		8	14
Interest		26	27
Expenses of consolidated VIEs		19	16
Inter-segment expenses	[2]	14	14
Total expenses		150	84
Income (loss) before income taxes		(165)	(54)
Provision (benefit) for income taxes		(57)	(23)
Net income (loss)		(108)	(31)
Identifiable assets		6,502	7,845
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	[2]	(30)	(34)
Total revenues		(30)	(34)
Inter-segment expenses	[2]	(30)	(32)
Total expenses		(30)	(32)
Income (loss) before income taxes		0	(2)
Provision (benefit) for income taxes		1	2
Net income (loss)		(1)	(4)
Identifiable assets	[3]	$ (2,948)	$ (2,840)

[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 of intercompany deferred income taxes, reinsurance balances and repurchase agreements.