Business Segments (Summary Of Company's Segment Results) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016		Mar. 31, 2015		Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	[1]	$ 115		$ 139
Net change in fair value of insured derivatives		(28)		28
Net gains (losses) on financial instruments at fair value and foreign exchange		(69)		30
Net investment losses related to other-than-temporary impairments		(1)		0
Net gains (losses) on extinguishment of debt		2
Other net realized gains (losses)		(1)		20
Revenues of consolidated VIEs		14		2
Inter-segment revenues	[2]	0		0
Total revenues		32		219
Losses and loss adjustment		22		(6)
Operating		45		48
Interest		50		50
Expenses of consolidated VIEs		16		14
Inter-segment expenses	[2]	0
Total expenses		133		106
Income (loss) before income taxes		(101)		113
Provision (benefit) for income taxes		(23)		44
Net income (loss)		(78)		69
Identifiable assets		12,751		15,895		$ 14,836
Operating Segments [Member] | U S Public Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	84		105
Net gains (losses) on financial instruments at fair value and foreign exchange		9		3
Net investment losses related to other-than-temporary impairments		0		0
Other net realized gains (losses)		0		(4)
Inter-segment revenues	[2]	6		10
Total revenues		99		114
Losses and loss adjustment		9		(6)
Operating		9		10
Inter-segment expenses	[2]	18		24
Total expenses		36		28
Income (loss) before income taxes		63		86
Provision (benefit) for income taxes		22		29
Net income (loss)		41		57
Identifiable assets		5,268		5,859
Operating Segments [Member] | Corporate Operations [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	5		5
Net gains (losses) on financial instruments at fair value and foreign exchange		(84)		32
Net investment losses related to other-than-temporary impairments		(1)
Net gains (losses) on extinguishment of debt		2
Other net realized gains (losses)		(1)		24
Inter-segment revenues	[2]	16		16
Total revenues		(63)		77
Operating		22		19
Interest		23		25
Inter-segment expenses	[2]	0		1
Total expenses		45		45
Income (loss) before income taxes		(108)		32
Provision (benefit) for income taxes		(24)		17
Net income (loss)		(84)		15
Identifiable assets		2,478		2,730
Operating Segments [Member] | International And Structured Finance Insurance [Member]
Segment Reporting Information [Line Items]
Revenues	[1]	26		29
Net change in fair value of insured derivatives		(28)		28
Net gains (losses) on financial instruments at fair value and foreign exchange		6		(5)
Other net realized gains (losses)				0
Revenues of consolidated VIEs		14		2
Inter-segment revenues	[2]	12		18
Total revenues		30		72
Losses and loss adjustment		13		0
Operating		14		19
Interest		27		25
Expenses of consolidated VIEs		16		14
Inter-segment expenses	[2]	14		18
Total expenses		84		76
Income (loss) before income taxes		(54)		(4)
Provision (benefit) for income taxes		(23)		(2)
Net income (loss)		(31)		(2)
Identifiable assets		7,845		10,565
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	[2]	(34)		(44)
Total revenues		(34)		(44)
Inter-segment expenses	[2]	(32)		(43)
Total expenses		(32)		(43)
Income (loss) before income taxes		(2)		(1)
Provision (benefit) for income taxes		2		0
Net income (loss)		(4)		(1)
Identifiable assets		$ (2,840)	[3]	$ (3,259)	[4]

[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 of intercompany deferred income taxes, reinsurance balances and repurchase agreements.

[4] Consists of intercompany reinsurance balances, repurchase agreements and deferred income taxes.