Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	[1]	$ 55	$ 77
Net change in fair value of insured derivatives		14	(5)
Net gains (losses) on financial instruments at fair value and foreign exchange		22	(9)
Net investment losses related to other-than-temporary impairments		(28)	(1)
Net gains (losses) on extinguishment of debt		0	0
Other net realized gains (losses)		1	(1)
Revenues of consolidated VIEs		(14)	12
Inter-segment revenues	[2]	0	0
Total revenues		50	73
Losses and loss adjustment		(38)	72
Operating		30	24
Interest		52	51
Expenses of consolidated VIEs		25	22
Inter-segment expenses	[2]	0	0
Total expenses		69	169
Income (loss) before income taxes		(19)	(96)
Identifiable assets		8,778	8,893	$ 8,076
Operating Segments [Member] | U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	38	53
Net change in fair value of insured derivatives		0	0
Net gains (losses) on financial instruments at fair value and foreign exchange		40	(6)
Net investment losses related to other-than-temporary impairments		(28)	(1)
Net gains (losses) on extinguishment of debt		0	0
Other net realized gains (losses)		1	0
Revenues of consolidated VIEs		(14)	0
Inter-segment revenues	[2]	7	7
Total revenues		44	53
Losses and loss adjustment		(50)	77
Operating		2	3
Interest		0	0
Expenses of consolidated VIEs		0	0
Inter-segment expenses	[2]	15	15
Total expenses		(33)	95
Income (loss) before income taxes		77	(42)
Identifiable assets		4,950	4,508
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	7	7
Net change in fair value of insured derivatives		0	0
Net gains (losses) on financial instruments at fair value and foreign exchange		(18)	3
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)		(1)	(2)
Revenues of consolidated VIEs		0	0
Inter-segment revenues	[2]	19	13
Total revenues		7	21
Losses and loss adjustment		0	0
Operating		22	13
Interest		19	20
Expenses of consolidated VIEs		0	0
Inter-segment expenses	[2]	6	6
Total expenses		47	39
Income (loss) before income taxes		(40)	(18)
Identifiable assets		1,195	1,159
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	10	17
Net change in fair value of insured derivatives		14	(5)
Net gains (losses) on financial instruments at fair value and foreign exchange		0	(6)
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)		1	1
Revenues of consolidated VIEs		0	12
Inter-segment revenues	[2]	3	7
Total revenues		28	26
Losses and loss adjustment		12	(5)
Operating		6	8
Interest		33	31
Expenses of consolidated VIEs		25	22
Inter-segment expenses	[2]	8	6
Total expenses		84	62
Income (loss) before income taxes		(56)	(36)
Identifiable assets		4,710	5,359
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	[2]	(29)	(27)
Total revenues		(29)	(27)
Inter-segment expenses	[2]	(29)	(27)
Total expenses		(29)	(27)
Income (loss) before income taxes		0	0
Identifiable assets	[3]	$ (2,077)	$ (2,133)

[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.