Expected Losses to be Paid (Net Exepcted Loss to be Paid by Accounting Model) (Details 3) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	$ 680		$ 1,104
Net Expected Loss to be Paid After Recoveries for R&W	677	[1]	1,106	[2]	1,065
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	455		124
Net Expected Loss Development After Recoveries for R&W	438		124
Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	451		745
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	459		158
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	(96)	[3]	(107)	[3]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(27)	[3]	44	[3]
Credit Default Swap
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	325	[1]	466	[1]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	23	[4]	(78)	[4]
Prime first lien
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	6		2
Net Expected Loss to be Paid After Recoveries for R&W	6	[1]	2	[2]	1
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	4		1
Net Expected Loss Development After Recoveries for R&W	4		1
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		2
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	2		0
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	[3]	0	[3]
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	2	[1]	0	[1]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	2	[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	315		295
Net Expected Loss to be Paid After Recoveries for R&W	315	[1]	295	[2]	400
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	62		(10)
Net Expected Loss Development After Recoveries for R&W	62		(10)
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	164		130
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	38		(2)
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	27	[3]	5	[3]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(10)	[3]	17	[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	[1]	160	[1]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	34	[4]	(25)	[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	(131)		210
Net Expected Loss to be Paid After Recoveries for R&W	(131)	[1]	210	[2]	629
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	39		7
Net Expected Loss Development After Recoveries for R&W	39		7
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	(114)		128
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	37		(94)
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	(37)	[3]	25	[3]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(8)	[3]	98	[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	20	[1]	57	[1]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	10	[4]	3	[4]
First mortgage, subprime
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	242		241
Net Expected Loss to be Paid After Recoveries for R&W	242	[1]	241	[2]	310
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	49		(47)
Net Expected Loss Development After Recoveries for R&W	49		(47)
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	118		96
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	31		(121)
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	50	[3]	44	[3]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	7	[3]	78	[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	74	[1]	101	[1]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	11	[4]	(4)	[4]
Total first lien
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	432		748
Net Expected Loss to be Paid After Recoveries for R&W	432	[1]	748	[2]	1,340
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	154		(49)
Net Expected Loss Development After Recoveries for R&W	154		(49)
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	172		356
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	108		(217)
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	40	[3]	74	[3]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(11)	[3]	193	[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	220	[1]	318	[1]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	57	[4]	(25)	[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	(39)		(86)
Net Expected Loss to be Paid After Recoveries for R&W	(39)	[1]	(86)	[2]	88
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(10)		(101)
Net Expected Loss Development After Recoveries for R&W	(10)		(101)
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	(60)		(58)
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	13		(96)
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	31	[3]	(22)	[3]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(23)	[3]	18	[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)	[1]	(6)	[1]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	0	[4]	(23)	[4]
HELOCs
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(111)		(31)
Net Expected Loss to be Paid After Recoveries for R&W	(111)	[1]	(31)	[2]	(806)
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	44		151
Net Expected Loss Development After Recoveries for R&W	44		151
HELOCs | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	56		128
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	37		318
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	(167)	[3]	(159)	[3]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	7	[3]	(167)	[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	[1]	0	[1]
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	(150)		(117)
Net Expected Loss to be Paid After Recoveries for R&W	(150)	[1]	(117)	[2]	(718)
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	34		50
Net Expected Loss Development After Recoveries for R&W	34		50
Second lien | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	(4)		70
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	50		222
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)	[3]	(181)	[3]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(16)	[3]	(149)	[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)	[1]	(6)	[1]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	0	[4]	(23)	[4]
U.S. RMBS
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	282		631
Net Expected Loss to be Paid After Recoveries for R&W	282	[1]	631	[2]	622
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	188		1
Net Expected Loss Development After Recoveries for R&W	188		1
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	168		426
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	158		5
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	(96)	[3]	(107)	[3]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(27)	[3]	44	[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	210	[1]	312	[1]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	57	[4]	(48)	[4]
TruPS
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	27		64
Net Expected Loss to be Paid After Recoveries for R&W	27	[1]	64	[2]	90
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(30)		(21)
Net Expected Loss Development After Recoveries for R&W	(30)		(21)
TruPS | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	1		13
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(11)		0
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	[3]	0	[3]
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	26	[1]	51	[1]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(19)	[4]	(21)	[4]
Structured finance
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	312		342
Net Expected Loss to be Paid After Recoveries for R&W	312	[1]	342	[2]	262
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	2		101
Net Expected Loss Development After Recoveries for R&W	2		101
Structured finance | Financial guaranty
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	224		240
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	15		111
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	[3]	0	[3]
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	88	[1]	102	[1]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(13)	[4]	(10)	[4]
US Public Finance
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W Excluding Other Lines of Business	7		16
Net Expected Loss to be Paid After Recoveries for R&W	7	[1]	16	[2]	82
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	74		(1)
Net Expected Loss Development After Recoveries for R&W	74		(1)
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	7		16
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	75		42
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	[3]	0	[3]
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	[1]	0	[1]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(1)	[4]	(43)	[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	52		51
Net Expected Loss to be Paid After Recoveries for R&W	52	[1]	51	[2]	7
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	221		44
Net Expected Loss Development After Recoveries for R&W	221		44
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	51		50
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	222		0
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	[3]	0	[3]
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]	1	[1]
Net Expected Loss Development After Recoveries for R&W Excluding Other Lines of Business	(1)	[4]	44	[4]
Other
Guarantor Obligations [Line Items]
Net Expected Loss to be Paid After Recoveries for R&W	(3)		2
Net Expected Loss Development After Recoveries for R&W	$ (17)		$ 0

[1]

[2] 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

[3] Refer to Note 10, Consolidation of Variable Interest Entities.

[4] Refer to Note 9, Financial Guaranty Contracts Accounted for as Credit Derivatives.