Expected Losses to be Paid (Net Exepcted Loss to be Paid by Accounting Model) (Details 3) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Dec. 31, 2012		Dec. 31, 2011
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	$ 500				$ 680
Net Expected Loss to be Paid After Recoveries for R&W	487		1,185		677	[1]	1,106
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(88)		212
Net Expected Loss Development After Recoveries for R&W	(98)		212
Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	310				451
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(72)		216
Financial Guaranty Variable Interest Entities
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(138)	[2]			(96)	[2]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(33)	[3]	(11)	[3]
Credit Default Swap
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	328				325
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	17	[4]	7	[4]
Prime first lien
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	11				6
Net Expected Loss to be Paid After Recoveries for R&W	11		2		6	[1]	2
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	6		0
Net Expected Loss Development After Recoveries for R&W	6		0
Prime first lien | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	4				4
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	0		1
Prime first lien | Financial Guaranty Variable Interest Entities
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	0	[2]			0	[2]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	0	[3]	0	[3]
Prime first lien | Credit Default Swap
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	7				2
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	6	[4]	(1)	[4]
Alt-A first lien
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	313				315
Net Expected Loss to be Paid After Recoveries for R&W	313		268		315	[1]	295
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	9		(2)
Net Expected Loss Development After Recoveries for R&W	9		(2)
Alt-A first lien | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	163				164
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	5		(4)
Alt-A first lien | Financial Guaranty Variable Interest Entities
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	26	[2]			27	[2]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(1)	[3]	(5)	[3]
Alt-A first lien | Credit Default Swap
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	124				124
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	5	[4]	7	[4]
First mortgage, option adjustable rate mortgage
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(327)				(131)
Net Expected Loss to be Paid After Recoveries for R&W	(327)		119		(131)	[1]	210
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(138)		(2)
Net Expected Loss Development After Recoveries for R&W	(138)		(2)
First mortgage, option adjustable rate mortgage | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(252)				(114)
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(93)		4
First mortgage, option adjustable rate mortgage | Financial Guaranty Variable Interest Entities
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(82)	[2]			(37)	[2]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(37)	[3]	(6)	[3]
First mortgage, option adjustable rate mortgage | Credit Default Swap
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	7				20
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(8)	[4]	0	[4]
First mortgage, subprime
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	263				242
Net Expected Loss to be Paid After Recoveries for R&W	263		248		242	[1]	241
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	25		16
Net Expected Loss Development After Recoveries for R&W	25		16
First mortgage, subprime | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	128				118
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	12		3
First mortgage, subprime | Financial Guaranty Variable Interest Entities
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	54	[2]			50	[2]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	4	[3]	8	[3]
First mortgage, subprime | Credit Default Swap
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	81				74
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	9	[4]	5	[4]
Total first lien
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	260				432
Net Expected Loss to be Paid After Recoveries for R&W	260		637		432	[1]	748
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(98)		12
Net Expected Loss Development After Recoveries for R&W	(98)		12
Total first lien | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	43				172
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(76)		4
Total first lien | Financial Guaranty Variable Interest Entities
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(2)	[2]			40	[2]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(34)	[3]	(3)	[3]
Total first lien | Credit Default Swap
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	219				220
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	12	[4]	11	[4]
Close ended second lien
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(21)				(39)
Net Expected Loss to be Paid After Recoveries for R&W	(21)		(101)		(39)	[1]	(86)
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	1		(3)
Net Expected Loss Development After Recoveries for R&W	1		(3)
Close ended second lien | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(38)				(60)
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	5		3
Close ended second lien | Financial Guaranty Variable Interest Entities
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	27	[2]			31	[2]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(3)	[3]	(4)	[3]
Close ended second lien | Credit Default Swap
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(10)				(10)
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(1)	[4]	(2)	[4]
HELOCs
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(122)				(111)
Net Expected Loss to be Paid After Recoveries for R&W	(122)		(43)		(111)	[1]	(31)
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(3)		8
Net Expected Loss Development After Recoveries for R&W	(3)		8
HELOCs | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	41				56
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(7)		12
HELOCs | Financial Guaranty Variable Interest Entities
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(163)	[2]			(167)	[2]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	4	[3]	(4)	[3]
HELOCs | Credit Default Swap
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	0				0
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	0	[4]	0	[4]
Second lien
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(143)				(150)
Net Expected Loss to be Paid After Recoveries for R&W	(143)		(144)		(150)	[1]	(117)
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(2)		5
Net Expected Loss Development After Recoveries for R&W	(2)		5
Second lien | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	3				(4)
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(2)		15
Second lien | Financial Guaranty Variable Interest Entities
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(136)	[2]			(136)	[2]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	1	[3]	(8)	[3]
Second lien | Credit Default Swap
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(10)				(10)
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(1)	[4]	(2)	[4]
U.S. RMBS
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	117				282
Net Expected Loss to be Paid After Recoveries for R&W	117		493		282	[1]	631
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(100)		17
Net Expected Loss Development After Recoveries for R&W	(100)		17
U.S. RMBS | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	46				168
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(78)		19
U.S. RMBS | Financial Guaranty Variable Interest Entities
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(138)	[2]			(96)	[2]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(33)	[3]	(11)	[3]
U.S. RMBS | Credit Default Swap
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	209				210
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	11	[4]	9	[4]
TruPS
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	23				27
Net Expected Loss to be Paid After Recoveries for R&W	23		58		27	[1]	64
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(3)		(4)
Net Expected Loss Development After Recoveries for R&W	(3)		(4)
TruPS | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	1				1
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	0		(5)
TruPS | Financial Guaranty Variable Interest Entities
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	0	[2]			0	[2]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	0	[3]	0	[3]
TruPS | Credit Default Swap
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	22				26
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(3)	[4]	1	[4]
Structured finance
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	307				312
Net Expected Loss to be Paid After Recoveries for R&W	307		296		312	[1]	342
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(2)		(22)
Net Expected Loss Development After Recoveries for R&W	(2)		(22)
Structured finance | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	211				224
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(10)		(19)
Structured finance | Financial Guaranty Variable Interest Entities
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	0	[2]			0	[2]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	0	[3]	0	[3]
Structured finance | Credit Default Swap
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	96				88
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	8	[4]	(3)	[4]
US Public Finance
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(9)				7
Net Expected Loss to be Paid After Recoveries for R&W	(9)		33		7	[1]	16
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	7		23
Net Expected Loss Development After Recoveries for R&W	7		23
US Public Finance | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(9)				7
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	7		23
US Public Finance | Financial Guaranty Variable Interest Entities
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	0	[2]			0	[2]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	0	[3]	0	[3]
US Public Finance | Credit Default Swap
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	0				0
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	0	[4]	0	[4]
Public finance Non-U.S.
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	62				52
Net Expected Loss to be Paid After Recoveries for R&W	62		303		52	[1]	51
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	10		198
Net Expected Loss Development After Recoveries for R&W	10		198
Public finance Non-U.S. | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	61				51
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	9		198
Public finance Non-U.S. | Financial Guaranty Variable Interest Entities
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	0	[2]			0	[2]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	0	[3]	0	[3]
Public finance Non-U.S. | Credit Default Swap
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	1				1
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	1	[4]	0	[4]
Other
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W	(13)				(3)
Net Expected Loss Development After Recoveries for R&W	$ (10)		$ 0

[1] Net of ceded paid losses, whether or not such amounts have been settled with reinsurers. Ceded paid losses are typically settled 45 days after the end of the reporting period. Such amounts are recorded in reinsurance recoverable on paid losses included in other assets.

[2] er to Note 9, Consolidation of Variable Interest Entities.

[3] Refer to Note 9, Consolidation of Variable Interest Entities.

[4] Refer to Note 8, Financial Guaranty Contracts Accounted for as Credit Derivatives.