Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	[1]	$ 47	$ 110	$ 155	$ 257
Revenues of consolidated VIEs		82	(12)	80	(72)
Inter-segment revenues	[2]	0	0	0	0
Total revenues		172	105	252	190
Operating		26	27	77	74
Expenses of consolidated VIEs		20	25	69	71
Inter-segment expenses	[2]	0	0	0	0
Total expenses		83	150	389	477
Income (loss) before income taxes		89	(45)	(137)	(287)
Identifiable assets		7,606	8,393	7,606	8,393	$ 8,107
Non Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Net change in fair value of insured derivatives		0	(1)	13	(13)
Net gains (losses) on financial instruments at fair value and foreign exchange		44	5	40	18
Net investment losses related to other-than-temporary impairments		0	(1)	(37)	(3)
Net gains (losses) on extinguishment of debt		(1)	3	(1)	3
Other net realized gains (losses)		0	1	2	0
Losses and loss adjustment		(13)	46	89	177
Interest		50	52	154	155
Operating Segments [Member] | U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	30	45	106	139
Revenues of consolidated VIEs		32	0	38	0
Inter-segment revenues	[2]	7	7	22	20
Total revenues		147	52	265	143
Operating		4	5	8	14
Expenses of consolidated VIEs		0	0	0	0
Inter-segment expenses	[2]	14	10	41	34
Total expenses		(72)	63	15	232
Income (loss) before income taxes		219	(11)	250	(89)
Identifiable assets		4,271	4,453	4,271	4,453
Operating Segments [Member] | U S Public Finance Insurance [Member] | Non Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Net change in fair value of insured derivatives			0	0	0
Net gains (losses) on financial instruments at fair value and foreign exchange		78	1	135	(13)
Net investment losses related to other-than-temporary impairments			(1)	(37)	(3)
Net gains (losses) on extinguishment of debt		0	0	0	0
Other net realized gains (losses)		0	0	1	0
Losses and loss adjustment		(90)	48	(34)	184
Interest		0	0	0	0
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	6	7	21	20
Revenues of consolidated VIEs		0	0	0	0
Inter-segment revenues	[2]	15	11	48	36
Total revenues		(1)	38	(5)	105
Operating		15	13	53	39
Expenses of consolidated VIEs		0	0	0	0
Inter-segment expenses	[2]	6	5	17	14
Total expenses		38	38	126	113
Income (loss) before income taxes		(39)	0	(131)	(8)
Identifiable assets		939	1,062	939	1,062
Operating Segments [Member] | Corporate Operations [Member] | Non Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Net change in fair value of insured derivatives			0	0	0
Net gains (losses) on financial instruments at fair value and foreign exchange		(20)	17	(71)	48
Net investment losses related to other-than-temporary impairments			0	0	0
Net gains (losses) on extinguishment of debt		(1)	3	(1)	3
Other net realized gains (losses)		(1)	0	(2)	(2)
Losses and loss adjustment		0	0	0	0
Interest		17	20	56	60
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	11	58	28	98
Revenues of consolidated VIEs		50	(12)	41	(72)
Inter-segment revenues	[2]	6	6	15	18
Total revenues		54	39	76	16
Operating		7	9	16	21
Expenses of consolidated VIEs		20	25	69	71
Inter-segment expenses	[2]	8	9	26	27
Total expenses		145	73	332	207
Income (loss) before income taxes		(91)	(34)	(256)	(191)
Identifiable assets		4,678	5,058	4,678	5,058
Operating Segments [Member] | International And Structured Finance Insurance [Member] | Non Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Net change in fair value of insured derivatives			(1)	13	(13)
Net gains (losses) on financial instruments at fair value and foreign exchange		(14)	(13)	(24)	(17)
Net investment losses related to other-than-temporary impairments			0	0	0
Net gains (losses) on extinguishment of debt		0	0	0	0
Other net realized gains (losses)		1	1	3	2
Losses and loss adjustment		77	(2)	123	(7)
Interest		33	32	98	95
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	0	0	0	0
Revenues of consolidated VIEs		0	0	1	0
Inter-segment revenues	[2]	(28)	(24)	(85)	(74)
Total revenues		(28)	(24)	(84)	(74)
Operating		0	0	0	0
Expenses of consolidated VIEs		0	0	0	0
Inter-segment expenses	[2]	(28)	(24)	(84)	(75)
Total expenses		(28)	(24)	(84)	(75)
Income (loss) before income taxes		0	0	0	1
Identifiable assets	[3]	(2,282)	(2,180)	(2,282)	(2,180)
Intersegment Elimination [Member] | Non Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Net change in fair value of insured derivatives			0	0	0
Net gains (losses) on financial instruments at fair value and foreign exchange		0	0	0	0
Net investment losses related to other-than-temporary impairments			0	0	0
Net gains (losses) on extinguishment of debt		0	0	0	0
Other net realized gains (losses)		0	0	0	0
Losses and loss adjustment		0	0	0	0
Interest		$ 0	$ 0	$ 0	$ 0

[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.