Expected Losses to be Paid - Components of R&W Development (Details) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013	Sep. 30, 2014		Sep. 30, 2013	Jun. 30, 2014	Dec. 31, 2013		Jun. 30, 2013	Dec. 31, 2012
Guarantor Obligations [Line Items]
Inclusion or removal of deals with breaches of R&W during period	$ 0		$ 0	$ 0		$ 6,000,000
Change in recovery assumptions as the result of recovery success	4,000,000		69,000,000	31,000,000		86,000,000
Estimated increase (decrease) in defaults that will result in additional (lower) breaches	(4,000,000)		13,000,000	(15,000,000)		10,000,000
Settlements and anticipated settlements	90,000,000		0	96,000,000		180,000,000
Accretion of discount on balance	3,000,000		4,000,000	48,000,000		12,000,000
Total	93,000,000		86,000,000	160,000,000		294,000,000
Current Net Par Covered	5,229,000,000			5,229,000,000
Net expected loss to be paid after recoveries for R&W	933,000,000	[1]	806,000,000	933,000,000	[1]	806,000,000	1,035,000,000	982,000,000	[1]	774,000,000	677,000,000
Loss and loss adjustment expense reserve	760,000,000			760,000,000				592,000,000
Net expected loss development after recoveries for R&W	(63,000,000)		(22,000,000)	(28,000,000)		(33,000,000)
Net par amount outstanding	418,015,000,000			418,015,000,000				459,107,000,000
Life insurance
Guarantor Obligations [Line Items]
Net expected loss development after recoveries for R&W	3,000,000			19,000,000
Prime first lien
Guarantor Obligations [Line Items]
Total	1,000,000		(1,000,000)	0		(1,000,000)
Net expected loss to be paid after recoveries for R&W	10,000,000	[1]	21,000,000	10,000,000	[1]	21,000,000	11,000,000	21,000,000	[1]	18,000,000	6,000,000
Net expected loss development after recoveries for R&W	(1,000,000)		3,000,000	(11,000,000)		16,000,000
Net par amount outstanding	488,000,000			488,000,000				541,000,000
Alt-A first lien
Guarantor Obligations [Line Items]
Total	19,000,000		37,000,000	20,000,000		24,000,000
Net expected loss to be paid after recoveries for R&W	248,000,000	[1]	206,000,000	248,000,000	[1]	206,000,000	301,000,000	304,000,000	[1]	288,000,000	315,000,000
Net expected loss development after recoveries for R&W	(18,000,000)		(85,000,000)	(6,000,000)		(83,000,000)
Net par amount outstanding	2,986,000,000			2,986,000,000				3,590,000,000
Option ARM
Guarantor Obligations [Line Items]
Total	10,000,000		40,000,000	30,000,000		206,000,000
Net expected loss to be paid after recoveries for R&W	(31,000,000)	[1]	7,000,000	(31,000,000)	[1]	7,000,000	(51,000,000)	(9,000,000)	[1]	(20,000,000)	(131,000,000)
Net expected loss development after recoveries for R&W	0		25,000,000	(39,000,000)		(92,000,000)
Net par amount outstanding	700,000,000			700,000,000				937,000,000
First mortgage, subprime
Guarantor Obligations [Line Items]
Total	5,000,000		7,000,000	34,000,000		6,000,000
Net expected loss to be paid after recoveries for R&W	307,000,000	[1]	303,000,000	307,000,000	[1]	303,000,000	341,000,000	304,000,000	[1]	274,000,000	242,000,000
Net expected loss development after recoveries for R&W	(11,000,000)		38,000,000	(12,000,000)		86,000,000
Net par amount outstanding	4,247,000,000			4,247,000,000				6,130,000,000
Closed end second lien and HELOCs
Guarantor Obligations [Line Items]
Total	(1,000,000)		1,000,000	(4,000,000)		(11,000,000)
HELOCs
Guarantor Obligations [Line Items]
Total	59,000,000		2,000,000	80,000,000		70,000,000
Net expected loss to be paid after recoveries for R&W	(148,000,000)	[1]	(129,000,000)	(148,000,000)	[1]	(129,000,000)	(117,000,000)	(116,000,000)	[1]	(97,000,000)	(111,000,000)
Net expected loss development after recoveries for R&W	(34,000,000)		(42,000,000)	(65,000,000)		(76,000,000)
Net par amount outstanding	1,817,000,000			1,817,000,000				2,279,000,000
Student loan
Guarantor Obligations [Line Items]
Current Net Par Covered	2,700,000,000			2,700,000,000
Net expected loss to be paid after recoveries for R&W	78,000,000			78,000,000
Net expected loss development after recoveries for R&W	5,000,000			13,000,000
Asset-backed student loan
Guarantor Obligations [Line Items]
Current Net Par Covered	1,900,000,000			1,900,000,000
Public finance student loan
Guarantor Obligations [Line Items]
Current Net Par Covered	800,000,000			800,000,000
Trust preferred securities collateralized debt obligations
Guarantor Obligations [Line Items]
Current Net Par Covered	4,500,000,000			4,500,000,000
Net expected loss to be paid after recoveries for R&W	26,000,000			26,000,000
Financial guaranty insurance contracts principal amount, percent in credit default swap form	71.00%			71.00%
Net expected loss development after recoveries for R&W	5,000,000			24,000,000
Public Finance City of Detroit
Guarantor Obligations [Line Items]
Recovery percentage				74.00%
Financial guaranty insurance contracts principal amount net	2,200,000,000			2,200,000,000
Public Finance Stockton General Fund
Guarantor Obligations [Line Items]
Financial guaranty insurance contracts principal amount net	117,000,000			117,000,000
Public Finance Luoisville Arena Authority
Guarantor Obligations [Line Items]
Financial guaranty insurance contracts principal amount net	338,000,000			338,000,000
Public Finance U.S.
Guarantor Obligations [Line Items]
Net expected loss to be paid after recoveries for R&W	333,000,000	[1]	183,000,000	333,000,000	[1]	183,000,000	339,000,000	264,000,000	[1]	71,000,000	7,000,000
Net expected loss development after recoveries for R&W	2,000,000		44,000,000	107,000,000		138,000,000
Net par amount outstanding	329,225,000,000			329,225,000,000				352,181,000,000
Spanish sub-sovereign debt
Guarantor Obligations [Line Items]
Financial guaranty insurance contracts principal amount	544,000,000			544,000,000
Financial guaranty insurance contracts principal amount net	390,000,000			390,000,000
Portugal sub-sovereign debt
Guarantor Obligations [Line Items]
Financial guaranty insurance contracts principal amount	120,000,000			120,000,000
Financial guaranty insurance contracts principal amount net	106,000,000			106,000,000
Hungarian-infrastructure bonds
Guarantor Obligations [Line Items]
Financial guaranty insurance contracts principal amount	555,000,000			555,000,000
Financial guaranty insurance contracts principal amount net	524,000,000			524,000,000
Spanish and Hungarian debt
Guarantor Obligations [Line Items]
Net expected loss to be paid after recoveries for R&W	50,000,000			50,000,000
Manufactured housing loans
Guarantor Obligations [Line Items]
Current Net Par Covered	231,000,000			231,000,000
Net expected loss to be paid after recoveries for R&W	25,000,000			25,000,000
Below investment grade (BIG)
Guarantor Obligations [Line Items]
Net par amount outstanding	19,646,000,000			19,646,000,000				22,538,000,000
Below investment grade (BIG) | Life insurance
Guarantor Obligations [Line Items]
Current Net Par Covered	2,700,000,000			2,700,000,000
Net expected loss to be paid after recoveries for R&W	90,000,000			90,000,000
Loss and loss adjustment expense reserve	598,000,000			598,000,000
Below investment grade (BIG) | Prime first lien
Guarantor Obligations [Line Items]
Net par amount outstanding	365,000,000			365,000,000				403,000,000
Below investment grade (BIG) | Alt-A first lien
Guarantor Obligations [Line Items]
Net par amount outstanding	1,912,000,000			1,912,000,000				2,728,000,000
Below investment grade (BIG) | Option ARM
Guarantor Obligations [Line Items]
Net par amount outstanding	194,000,000			194,000,000				598,000,000
Below investment grade (BIG) | First mortgage, subprime
Guarantor Obligations [Line Items]
Net par amount outstanding	1,663,000,000			1,663,000,000				1,945,000,000
Below investment grade (BIG) | HELOCs
Guarantor Obligations [Line Items]
Net par amount outstanding	1,606,000,000			1,606,000,000				1,897,000,000
Below investment grade (BIG) | Asset-backed student loan
Guarantor Obligations [Line Items]
Current Net Par Covered	198,000,000			198,000,000
Below investment grade (BIG) | Public finance student loan
Guarantor Obligations [Line Items]
Current Net Par Covered	244,000,000			244,000,000
Below investment grade (BIG) | Trust preferred securities collateralized debt obligations
Guarantor Obligations [Line Items]
Current Net Par Covered	1,500,000,000			1,500,000,000
Below investment grade (BIG) | Public Finance U.S.
Guarantor Obligations [Line Items]
Net par amount outstanding	8,542,000,000			8,542,000,000				9,094,000,000
Below investment grade (BIG) | Manufactured housing loans
Guarantor Obligations [Line Items]
Current Net Par Covered	164,000,000			164,000,000
Below investment grade (BIG) | Ballantyne Re Plc and Orkney Re I I Plc | Life insurance
Guarantor Obligations [Line Items]
Number of transactions insured	2			2
Sewer | Public Finance City of Detroit
Guarantor Obligations [Line Items]
Current Net Par Covered	1,100,000,000			1,100,000,000
Water Sector | Public Finance City of Detroit
Guarantor Obligations [Line Items]
Current Net Par Covered	882,000,000			882,000,000
Sewer and Water | Public Finance City of Detroit
Guarantor Obligations [Line Items]
Current Net Par Covered	1,800,000,000			1,800,000,000
Remaining principle amount after tender offer	841,000,000			841,000,000
Bonds sold in tender offer, amount	677,000,000			677,000,000
Municipal Bonds State General Obligation | Public Finance City of Detroit
Guarantor Obligations [Line Items]
Current Net Par Covered	128,000,000			128,000,000
City Certificates of Participation | Public Finance City of Detroit
Guarantor Obligations [Line Items]
Current Net Par Covered	175,000,000			175,000,000
Refinancing risk on infrastructure transactions
Guarantor Obligations [Line Items]
Current Net Par Covered	3,000,000,000			3,000,000,000
Number of infrastructure transactions				2
Financial insurance guaranty, infrastructure finance, potential liability	1,800,000,000			1,800,000,000
Refinancing risk on infrastructure transactions | Minimum
Guarantor Obligations [Line Items]
Estimated years for recoveries on infrastructure transactions				10 years
Refinancing risk on infrastructure transactions | Maximum
Guarantor Obligations [Line Items]
Estimated years for recoveries on infrastructure transactions				35 years
Puerto Rico
Guarantor Obligations [Line Items]
Net par amount outstanding	4,900,000,000			4,900,000,000
Puerto Rico | Below investment grade (BIG)
Guarantor Obligations [Line Items]
Net par amount outstanding	4,672,000,000			4,672,000,000
Puerto Rico | Below investment grade (BIG) | Puerto Rico
Guarantor Obligations [Line Items]
Net par amount outstanding	$ 4,700,000,000			$ 4,700,000,000

[1] Includes expected loss adjustment expenses ("LAE") to be paid of $28 million as of September 30, 2014 and $34 million as of December 31, 2013. The Company paid $6 million and $12 million in LAE for Third Quarter 2014 and 2013, respectively, and $20 million and $41 million in LAE for Nine Months 2014 and 2013, respectively.