Expected Losses to be Paid (Other Significant Disclosures and Transactions) (Details 9)			3 Months Ended				1 Months Ended	3 Months Ended			3 Months Ended
	Mar. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Mar. 31, 2013 Refinancing Risk on Three Largest Infrastructure Transactions [Member] USD ($) Transaction	Mar. 31, 2013 Refinancing Risk on Three Largest Infrastructure Transactions [Member] Minimum	Mar. 31, 2013 Refinancing Risk on Three Largest Infrastructure Transactions [Member] Maximum	Mar. 31, 2013 Refinancing Risk on Three Largest Infrastructure Transactions [Member] Amount Payable Between 2014 and 2020 USD ($)	Feb. 28, 2013 Flagstar [Member] USD ($)	Mar. 31, 2013 Student Loan [Member] USD ($)	Mar. 31, 2013 Asset-backed Student Loan [Member] USD ($)	Mar. 31, 2013 Public Finance Student Loan [Member] USD ($)	Mar. 31, 2013 Trust Preferred Securities Collateralized Debt Obligations USD ($)	Mar. 31, 2013 Life Insurance [Member] USD ($)	Mar. 31, 2013 Public Finance Jefferson County Alabama Sewer Authority [Member] USD ($)	Mar. 31, 2013 Public Finance Jefferson County Alabama Sewer Authority Bonds [Member] USD ($)	Mar. 31, 2013 Public Finance Jefferson County Alabama Sewer Authority Warrants [Member] USD ($)	Mar. 31, 2013 Public Finance Stockton General Fund USD ($)	Mar. 31, 2013 Public Finance Harrisburg Pennsylvania [Member] USD ($)	Mar. 31, 2013 Public Finance Luoisville Arena Authority [Member] USD ($)	Mar. 31, 2013 Public Finance bonds secured by excess free cash flows of Foxwoods Casino [Member] USD ($)	Mar. 31, 2013 Troubled Municipal Credits [Member] USD ($)	Dec. 31, 2012 Troubled Municipal Credits [Member] USD ($)	Mar. 31, 2013 Spanish sub-sovereign debt [Member] EUR (€)	Mar. 31, 2013 Hungarian-infrastructure bonds [Member] USD ($)	Mar. 31, 2013 Spanish and Hungarian debt [Member] USD ($)	Dec. 31, 2012 Spanish and Hungarian debt [Member] USD ($)	Mar. 31, 2013 Manufactured Housing Loans [Member] USD ($)	Mar. 31, 2013 Below investment grade (BIG) USD ($)	Dec. 31, 2012 Below investment grade (BIG) USD ($)	Mar. 31, 2013 Below investment grade (BIG) Asset-backed Student Loan [Member] USD ($)	Mar. 31, 2013 Below investment grade (BIG) Public Finance Student Loan [Member] USD ($)	Mar. 31, 2013 Below investment grade (BIG) Trust Preferred Securities Collateralized Debt Obligations USD ($)	Mar. 31, 2013 Below investment grade (BIG) Life Insurance [Member] USD ($)	Mar. 31, 2013 Below investment grade (BIG) Public Finance Harrisburg Pennsylvania [Member] USD ($)	Mar. 31, 2013 Below investment grade (BIG) Hungarian-infrastructure bonds [Member] USD ($)	Mar. 31, 2013 Below investment grade (BIG) Manufactured Housing Loans [Member] USD ($)	Mar. 31, 2013 Ballantyne Re Plc and Orkney Re II Plc Below investment grade (BIG) Life Insurance [Member] Transaction	Mar. 31, 2013 Bank of America Agreement USD ($)	Apr. 14, 2011 Bank of America Agreement USD ($)
Guarantor Obligations [Line Items]
Net Par Outstanding Financial Guaranty Insurance								$ 2,900,000,000	$ 1,900,000,000	$ 1,000,000,000	$ 5,500,000,000		$ 464,000,000	$ 192,000,000	$ 272,000,000											$ 288,000,000			$ 195,000,000	$ 326,000,000	$ 2,800,000,000	$ 2,800,000,000			$ 198,000,000
Percent of net par outstanding financial guaranty insurance in credit default swap form											72.00%
Guaranty Liabilities															32,000,000
Projected Net Expected Loss								55,000,000			23,000,000		188,000,000							9,000,000	7,000,000											128,000,000
Exposure of Debt																						453,000,000	648,000,000
Net exposure of debt													684,000,000			158,000,000		336,000,000				329,000,000	612,000,000											576,000,000
Expected LAE for mitigating claim liabilities	31,000,000	39,000,000																						49,000,000	39,000,000
Claims Paid, Net																11,000,000
Liability for Unpaid Claims and Claims Adjustment Expense Net Largest Single Loss								25,000,000																		33,000,000
Number of Infrastructure Transactions			3
Financial Insurance Guaranty, Infrastructure Finance, Potential Liability			3,800,000,000			3,000,000,000
Estimated Years for Liabilities to be Incurred on Infrastructure Transactions			13 years
Financial Insurance Guaranty, Infrastructure Finance, Possible Claims Requiring Payment			1,400,000,000
Estimated Years for Recoveries on Infrastructure Transactions				10 years	35 years
Damages awarded							90,700,000
Pre-judgment interest							15,900,000
Increase (Decrease) in Projected Expected Loss to be Paid								1,000,000			4,000,000	11,000,000
Loss and loss adjustment expense reserve	532,000,000	601,000,000															154,000,000		26,000,000													923,000,000	92,000,000
Net Claims to Date																	15,000,000		89,000,000
Number of transactions insured																																				2
Maximum Loss Up To Which Loss Sharing Percentage Applicable																																						6,600,000,000
Collateral Losses																																					3,300,000,000
Insured financial obligations gross outstanding principal amount																																					3,300,000,000
Insured Financial Obligations Outstanding Principal Amount	501,817,000,000	519,893,000,000																									22,354,000,000	23,392,000,000									3,100,000,000
Financial Guaranty Insurance Contracts Total Recoveries Under Negotiated Agreement	1,700,000,000
Loss Sharing Percentage																																						80.00%
Judgment granted in favor of AGM							$ 106,500,000