Expected Losses to be Paid (Details 9)	3 Months Ended				9 Months Ended					3 Months Ended		3 Months Ended						9 Months Ended				3 Months Ended					3 Months Ended		3 Months Ended				9 Months Ended					3 Months Ended																											1 Months Ended
	Sep. 30, 2013 USD ($)		Sep. 30, 2012 USD ($)		Sep. 30, 2013 USD ($)		Sep. 30, 2012 USD ($)		Dec. 31, 2012 USD ($)	Sep. 30, 2013 Refinancing Risk on Three Largest Infrastructure Transactions [Member] AUD Transaction	Sep. 30, 2013 Refinancing Risk on Three Largest Infrastructure Transactions [Member] USD ($)	Sep. 30, 2013 Refinancing Risk on Three Largest Infrastructure Transactions [Member] Minimum	Sep. 30, 2013 Refinancing Risk on Three Largest Infrastructure Transactions [Member] Maximum	Sep. 30, 2013 Second lien USD ($)		Sep. 30, 2012 Second lien USD ($)		Sep. 30, 2013 Second lien USD ($)		Sep. 30, 2012 Second lien USD ($)		Sep. 30, 2013 Student Loan [Member] USD ($)	Sep. 30, 2013 Asset-backed Student Loan [Member] USD ($)	Sep. 30, 2013 Asset-backed Student Loan [Member] Below investment grade (BIG) USD ($)	Sep. 30, 2013 Public Finance Student Loan [Member] USD ($)	Sep. 30, 2013 Public Finance Student Loan [Member] Below investment grade (BIG) USD ($)	Sep. 30, 2013 Trust Preferred Securities Collateralized Debt Obligations USD ($)	Sep. 30, 2013 Trust Preferred Securities Collateralized Debt Obligations Below investment grade (BIG) USD ($)	Sep. 30, 2013 TruPS USD ($)		Sep. 30, 2012 TruPS USD ($)		Sep. 30, 2013 TruPS USD ($)		Sep. 30, 2012 TruPS USD ($)		Sep. 30, 2013 Public Finance City of Detroit [Member] USD ($)	Sep. 30, 2013 Life Insurance [Member] USD ($)	Sep. 30, 2013 Life Insurance [Member] Below investment grade (BIG) USD ($)	Sep. 30, 2013 Public Finance Jefferson County Alabama Sewer Authority [Member] USD ($)	Sep. 30, 2013 Public Finance Jefferson County Alabama Sewer Authority Bonds [Member] USD ($)	Sep. 30, 2013 Public Finance Jefferson County Alabama Sewer Authority Warrants [Member] USD ($)	Sep. 30, 2013 Public Finance Stockton General Fund USD ($)	Sep. 30, 2013 Public Finance Harrisburg Pennsylvania [Member] USD ($)	Sep. 30, 2013 Public Finance Harrisburg Pennsylvania [Member] Below investment grade (BIG) USD ($)	Sep. 30, 2013 Public Finance Luoisville Arena Authority [Member] USD ($)	Sep. 30, 2013 Public Finance bonds secured by excess free cash flows of Foxwoods Casino [Member] USD ($)	Sep. 30, 2013 Troubled Municipal Credits [Member] USD ($)	Jun. 30, 2013 Troubled Municipal Credits [Member] USD ($)	Sep. 30, 2013 Spanish sub-sovereign debt [Member] EUR (€)	Sep. 30, 2013 Portugal sub-sovereign debt [Member] EUR (€)	Sep. 30, 2013 Hungarian-infrastructure bonds [Member] USD ($)	Sep. 30, 2013 Hungarian-infrastructure bonds [Member] Below investment grade (BIG) USD ($)	Sep. 30, 2013 Spanish and Hungarian debt [Member] USD ($)	Jun. 30, 2013 Spanish and Hungarian debt [Member] USD ($)	Sep. 30, 2013 Manufactured Housing Loans [Member] USD ($)	Jun. 30, 2013 Manufactured Housing Loans [Member] USD ($)	Sep. 30, 2013 Manufactured Housing Loans [Member] Below investment grade (BIG) USD ($)	Sep. 30, 2013 Ballantyne Re Plc and Orkney Re II Plc Life Insurance [Member] Below investment grade (BIG) Transaction	Sep. 30, 2013 Bank of America Agreement Second lien USD ($)	Sep. 30, 2013 Water and Sewer [Member] Public Finance City of Detroit [Member] USD ($)	Sep. 30, 2013 Water Sector [Member] Public Finance City of Detroit [Member] USD ($)	Sep. 30, 2013 Municipal Bonds State General Obligation [Member] Public Finance City of Detroit [Member] USD ($)	Sep. 30, 2013 City Certificates of Participation [Member] Public Finance City of Detroit [Member] USD ($)	Oct. 31, 2013 AGM Charles E. Wilson vs. JPMorgan Chase & Co et al [Member] Pending Litigation [Member] Subsequent Event [Member] USD ($)
Guarantor Obligations [Line Items]
Net par outstanding amount	$ 6,002,000,000				$ 6,002,000,000						$ 3,100,000,000											$ 2,800,000,000	$ 1,900,000,000	$ 209,000,000	$ 900,000,000	$ 257,000,000	$ 5,200,000,000	$ 2,000,000,000											$ 2,800,000,000	$ 464,000,000	$ 192,000,000	$ 272,000,000														$ 267,000,000		$ 185,000,000		$ 1,426,000,000	$ 1,000,000,000	$ 784,000,000	$ 146,000,000	$ 175,000,000
Loss and loss adjustment expense reserve	601,000,000				601,000,000				601,000,000																														623,000,000					155,000,000	92,000,000
Number of transactions insured																																																											2
Projected Net Expected Loss																						64,000,000					50,000,000												85,000,000	187,000,000								183,000,000	71,000,000
Increase (Decrease) in Projected Expected Loss to be Paid										413,000,000												8,000,000					17,000,000											30,000,000
Liability for Unpaid Claims and Claims Adjustment Expense Net Largest Single Loss																						27,000,000																																		26,000,000	30,000,000
Percent of net par outstanding financial guaranty insurance in credit default swap form																											71.00%
Liability For Unpaid Claims And Claims Adjustment Expense Claims Paid After Recoveries for RW	(54,000,000)	[1]	509,000,000	[1]	(162,000,000)	[1]	718,000,000	[1]						(11,000,000)	[1]	30,000,000	[1]	(77,000,000)	[1]	24,000,000	[1]								(8,000,000)	[1]	2,000,000	[1]	(16,000,000)	[1]	5,000,000	[1]
Net exposure of debt																																					2,100,000,000			681,000,000			119,000,000			337,000,000	145,000,000			327,000,000	146,000,000	637,000,000	599,000,000
Claims Paid, Net																																											25,000,000
Guaranty Liabilities																																										30,000,000
Net Claims to Date																																												17,000,000			116,000,000
Exposure of Debt																																																		450,000,000	159,000,000	673,000,000
Expected LAE for mitigating claim liabilities	34,000,000				34,000,000				39,000,000																																													48,000,000	49,000,000
Number of Infrastructure Transactions										2
Financial Insurance Guaranty, Infrastructure Finance, Potential Liability											1,800,000,000
Financial Insurance Guaranty, Infrastructure Finance, Possible Claims Requiring Payment											1,300,000,000
Estimated Years for Recoveries on Infrastructure Transactions												10 years	35 years
Litigation Case, Voting Class, Aggregate Claims Amount, Accept Vote																																																																	3,900,000,000
Litigation Case, Voting Class, Aggregate Claims Amount, Reject Vote																																																																	$ 18,000,000

[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.