Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	[1]	$ 53	$ 70	$ 108	$ 147
Net change in fair value of insured derivatives		(1)	(7)	13	(12)
Net gains (losses) on financial instruments at fair value and foreign exchange		(26)	22	(4)	13
Net investment losses related to other-than-temporary impairments		(9)	(1)	(37)	(2)
Other net realized gains (losses)		1		2	(1)
Revenues of consolidated VIEs		12	(72)	(2)	(60)
Inter-segment revenues	[2]	0	0	0	0
Total revenues		30	12	80	85
Losses and loss adjustment		140	59	102	131
Operating		21	23	51	47
Interest		52	52	104	103
Expenses of consolidated VIEs		24	24	49	46
Inter-segment expenses	[2]	0	0	0	0
Total expenses		237	158	306	327
Income (loss) before income taxes		(207)	(146)	(226)	(242)
Identifiable assets		8,647	8,730	8,647	8,730	$ 8,107
Operating Segments [Member] | U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	38	41	76	94
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	(8)	57	(14)
Net investment losses related to other-than-temporary impairments		(9)	(1)	(37)	(2)
Other net realized gains (losses)		0		1	0
Revenues of consolidated VIEs		20	0	6	0
Inter-segment revenues	[2]	8	6	15	13
Total revenues		74	38	118	91
Losses and loss adjustment		106	59	56	136
Operating		2	6	4	9
Interest		0	0	0	0
Expenses of consolidated VIEs		0	0	0	0
Inter-segment expenses	[2]	12	9	27	24
Total expenses		120	74	87	169
Income (loss) before income taxes		(46)	(36)	31	(78)
Identifiable assets		5,011	4,412	5,011	4,412
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	8	7	15	14
Net change in fair value of insured derivatives		0	0	0	0
Net gains (losses) on financial instruments at fair value and foreign exchange		(33)	28	(51)	31
Net investment losses related to other-than-temporary impairments		0	0	0	0
Other net realized gains (losses)		0		(1)	(2)
Revenues of consolidated VIEs		0	0	0	0
Inter-segment revenues	[2]	14	11	33	24
Total revenues		(11)	46	(4)	67
Losses and loss adjustment		0	0	0	0
Operating		16	13	38	26
Interest		20	20	39	40
Expenses of consolidated VIEs		0	0	0	0
Inter-segment expenses	[2]	5	3	11	9
Total expenses		41	36	88	75
Income (loss) before income taxes		(52)	10	(92)	(8)
Identifiable assets		1,123	1,100	1,123	1,100
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	7	22	17	39
Net change in fair value of insured derivatives		(1)	(7)	13	(12)
Net gains (losses) on financial instruments at fair value and foreign exchange		(10)	2	(10)	(4)
Net investment losses related to other-than-temporary impairments		0	0	0	0
Other net realized gains (losses)		1		2	1
Revenues of consolidated VIEs		(9)	(72)	(9)	(60)
Inter-segment revenues	[2]	6	6	9	13
Total revenues		(6)	(49)	22	(23)
Losses and loss adjustment		34	0	46	(5)
Operating		3	4	9	12
Interest		32	32	65	63
Expenses of consolidated VIEs		24	24	49	46
Inter-segment expenses	[2]	10	12	18	18
Total expenses		103	72	187	134
Income (loss) before income taxes		(109)	(121)	(165)	(157)
Identifiable assets		4,678	5,248	4,678	5,248
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	0	0	0	0
Net change in fair value of insured derivatives		0	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	0
Other net realized gains (losses)		0		0	0
Revenues of consolidated VIEs		1	0	1	0
Inter-segment revenues	[2]	(28)	(23)	57	(50)
Total revenues		(27)	(23)	(56)	(50)
Losses and loss adjustment		0	0	0	0
Operating		0	0	0	0
Interest		0	0	0	0
Expenses of consolidated VIEs		0	0	0	0
Inter-segment expenses	[2]	(27)	(24)	(56)	(51)
Total expenses		(27)	(24)	(56)	(51)
Income (loss) before income taxes		0	1		1
Identifiable assets	[3]	$ (2,165)	$ (2,030)	$ (2,165)	$ (2,030)

[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.