Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total revenues		$ (6)	$ 50
Total expenses		327	69
Income (loss) before income taxes		(333)	(19)
Identifiable assets		6,632		$ 7,284
Non Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	43	55
Net change in fair value of insured derivatives			14
Net gains (losses) on financial instruments at fair value and foreign exchange		(63)	22
Net investment losses related to other-than-temporary impairments		0	(28)
Other net realized gains (losses)		0	1
Inter-segment revenues	[2]	0	0
Losses and loss adjustment		243	(38)
Operating		20	30
Interest		47	52
Inter-segment expenses	[2]	0	0
Variable Interest Entity Primary Beneficiary [Member]
Segment Reporting Information [Line Items]
Net gains (losses) on financial instruments at fair value and foreign exchange		15	18
Other net realized gains (losses)		(9)	(42)
Revenues of consolidated VIEs		14	(14)
Interest		15	22
Expenses of consolidated VIEs		17	25
Identifiable assets		1,200		$ 1,600
Operating Segments [Member] | U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Net change in fair value of insured derivatives			0
Inter-segment revenues	[2]	7	7
Total revenues		19	44
Inter-segment expenses	[2]	14	15
Total expenses		64	(33)
Income (loss) before income taxes		(45)	77
Identifiable assets		3,867
Operating Segments [Member] | U S Public Finance Insurance [Member] | Non Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	30	38
Net gains (losses) on financial instruments at fair value and foreign exchange		(18)	40
Net investment losses related to other-than-temporary impairments			(28)
Other net realized gains (losses)			1
Losses and loss adjustment		48	(50)
Operating		2	2
Interest		0	0
Operating Segments [Member] | U S Public Finance Insurance [Member] | Variable Interest Entity Primary Beneficiary [Member]
Segment Reporting Information [Line Items]
Revenues of consolidated VIEs		0	(14)
Expenses of consolidated VIEs		0	0
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Net change in fair value of insured derivatives			0
Net investment losses related to other-than-temporary impairments			0
Inter-segment revenues	[2]	18	19
Total revenues		(32)	7
Inter-segment expenses	[2]	6	6
Total expenses		36	47
Income (loss) before income taxes		(68)	(40)
Identifiable assets		1,000
Operating Segments [Member] | Corporate Operations [Member] | Non Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	6	7
Net gains (losses) on financial instruments at fair value and foreign exchange		(56)	(18)
Other net realized gains (losses)			(1)
Losses and loss adjustment		0	0
Operating		14	22
Interest		16	19
Operating Segments [Member] | Corporate Operations [Member] | Variable Interest Entity Primary Beneficiary [Member]
Segment Reporting Information [Line Items]
Revenues of consolidated VIEs		0	0
Expenses of consolidated VIEs		0	0
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Net investment losses related to other-than-temporary impairments			0
Inter-segment revenues	[2]	4	3
Total revenues		36	28
Inter-segment expenses	[2]	9	8
Total expenses		256	84
Income (loss) before income taxes		(220)	(56)
Identifiable assets		4,051
Operating Segments [Member] | International And Structured Finance Insurance [Member] | Non Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	7	10
Net change in fair value of insured derivatives			14
Net gains (losses) on financial instruments at fair value and foreign exchange		11	0
Other net realized gains (losses)			1
Losses and loss adjustment		195	12
Operating		4	6
Interest		31	33
Operating Segments [Member] | International And Structured Finance Insurance [Member] | Variable Interest Entity Primary Beneficiary [Member]
Segment Reporting Information [Line Items]
Revenues of consolidated VIEs		14	0
Expenses of consolidated VIEs		17	25
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Other net realized gains (losses)			0
Inter-segment revenues	[2]	(29)	(29)
Total revenues		(29)	(29)
Inter-segment expenses	[2]	(29)	(29)
Total expenses		(29)	(29)
Income (loss) before income taxes		0	0
Identifiable assets	[3]	(2,286)
Intersegment Elimination [Member] | Non Variable Interest Entity [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	0	0
Net change in fair value of insured derivatives			0
Net gains (losses) on financial instruments at fair value and foreign exchange		0	0
Net investment losses related to other-than-temporary impairments			0
Losses and loss adjustment		0	0
Operating		0	0
Interest		0	0
Intersegment Elimination [Member] | Variable Interest Entity Primary Beneficiary [Member]
Segment Reporting Information [Line Items]
Revenues of consolidated VIEs		0	0
Expenses of consolidated VIEs		$ 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.