Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2022		Mar. 31, 2021		Dec. 31, 2021
Segment Reporting Information [Line Items]
Revenues		$ 27	[1]	$ 34	[2]
Net gains (losses) on financial instruments at fair value and foreign exchange		17		52
Revenues of consolidated VIEs		(4)		(14)
Inter-segment revenues	[3]	0		0
Total revenues		40		72
Losses and loss adjustment		49		98
Amortization of deferred acquisition costs and operating		21		28
Interest		41		41
Expenses of consolidated VIEs		2		11
Inter-segment expenses	[3]	0		0
Total expenses		113		178
Income (loss) before income taxes		(73)		(106)
Identifiable assets		4,443				$ 4,696
Operating Segments [Member] | US Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues		20	[1]	22	[2]
Net gains (losses) on financial instruments at fair value and foreign exchange		(16)		(2)
Revenues of consolidated VIEs		0		0
Inter-segment revenues	[3]	8		8
Total revenues		12		28
Losses and loss adjustment		87		109
Amortization of deferred acquisition costs and operating		3		4
Interest		0		0
Expenses of consolidated VIEs		0		0
Inter-segment expenses	[3]	13		14
Total expenses		103		127
Income (loss) before income taxes		(91)		(99)
Identifiable assets		3,196
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Revenues		2	[1]	5	[2]
Net gains (losses) on financial instruments at fair value and foreign exchange		39		55
Revenues of consolidated VIEs		0		0
Inter-segment revenues	[3]	17		18
Total revenues		58		78
Losses and loss adjustment		0		0
Amortization of deferred acquisition costs and operating		15		21
Interest		14		14
Expenses of consolidated VIEs		0		0
Inter-segment expenses	[3]	6		6
Total expenses		35		41
Income (loss) before income taxes		23		37
Identifiable assets		798
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues		5	[1]	7	[2]
Net gains (losses) on financial instruments at fair value and foreign exchange		(6)		(1)
Revenues of consolidated VIEs		(4)		(14)
Inter-segment revenues	[3]	3		4
Total revenues		(2)		(4)
Losses and loss adjustment		(38)		(11)
Amortization of deferred acquisition costs and operating		3		3
Interest		27		27
Expenses of consolidated VIEs		2		11
Inter-segment expenses	[3]	9		10
Total expenses		3		40
Income (loss) before income taxes		(5)		(44)
Identifiable assets		1,877
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues		0	[1]	0	[2]
Net gains (losses) on financial instruments at fair value and foreign exchange		0		0
Revenues of consolidated VIEs		0		0
Inter-segment revenues	[3]	(28)		(30)
Total revenues		(28)		(30)
Losses and loss adjustment		0		0
Amortization of deferred acquisition costs and operating		0		0
Interest		0		0
Expenses of consolidated VIEs		0		0
Inter-segment expenses	[3]	(28)		(30)
Total expenses		(28)		(30)
Income (loss) before income taxes		0		$ 0
Identifiable assets	[4]	$ (1,428)

[1] Consists of net premiums earned, net investment income, net realized investment gains (losses), fees and reimbursements and other net realized gains (losses).

[2] Consists of net premiums earned, net investment income, net realized investment gains (losses) and fees and reimbursements.

[3] Primarily represents intercompany service charges and intercompany net investment income and expenses.

[4] Consists principally of intercompany reinsurance balances.