Expected Losses to be Paid (Other Significant Disclosures and Transactions) (Details 9)			12 Months Ended			1 Months Ended	12 Months Ended			12 Months Ended												12 Months Ended													1 Months Ended	12 Months Ended
	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2012 Refinancing Risk on Three Largest Infrastructure Transactions [Member] USD ($) Transaction	Dec. 31, 2012 Refinancing Risk on Three Largest Infrastructure Transactions [Member] Minimum	Dec. 31, 2012 Refinancing Risk on Three Largest Infrastructure Transactions [Member] Maximum	Feb. 28, 2013 Flagstar [Member] USD ($)	Dec. 31, 2012 Student Loan [Member] USD ($)	Dec. 31, 2012 Asset-backed Student Loan [Member] USD ($)	Dec. 31, 2012 Public Finance Student Loan [Member] USD ($)	Dec. 31, 2012 Trust Preferred Securities Collateralized Debt Obligations [Member] USD ($)	Dec. 31, 2012 Public Finance Jefferson County Alabama Sewer Authority [Member] USD ($)	Dec. 31, 2012 Public Finance Jefferson County Alabama Sewer Authority Bonds [Member] USD ($)	Dec. 31, 2012 Public Finance Jefferson County Alabama Sewer Authority Warrants [Member] USD ($)	Dec. 31, 2012 Public Finance Stockton General Fund USD ($)	Dec. 31, 2012 Public Finance Harrisburg Pennsylvania [Member] USD ($)	Dec. 31, 2012 Public Finance Luoisville Arena Authority [Member] USD ($)	Dec. 31, 2012 Public Finance bonds secured by excess free cash flows of Foxwoods Casino [Member] USD ($)	Dec. 31, 2012 Troubled Municipal Credits [Member] USD ($)	Dec. 31, 2011 Troubled Municipal Credits [Member] USD ($)	Jun. 30, 2012 Sovereign Debt USD ($)	Jun. 30, 2012 Sovereign Debt EUR (€)	Dec. 31, 2012 Spanish sub-sovereign debt [Member] USD ($)	Dec. 31, 2012 Spanish sub-sovereign debt [Member] EUR (€)	Dec. 31, 2012 Manufactured Housing Loans [Member] USD ($)	Dec. 31, 2011 Manufactured Housing Loans [Member] USD ($)	Dec. 31, 2012 Below investment grade (BIG) USD ($)	Dec. 31, 2011 Below investment grade (BIG) USD ($)	Dec. 31, 2012 Below investment grade (BIG) Asset-backed Student Loan [Member] USD ($)	Dec. 31, 2012 Below investment grade (BIG) Public Finance Student Loan [Member] USD ($)	Dec. 31, 2012 Below investment grade (BIG) Trust Preferred Securities Collateralized Debt Obligations [Member] USD ($)	Dec. 31, 2012 Below investment grade (BIG) Life Insurance [Member] USD ($)	Dec. 31, 2012 Below investment grade (BIG) Public Finance Harrisburg Pennsylvania [Member] USD ($)	Dec. 31, 2012 Below investment grade (BIG) Manufactured Housing Loans [Member] USD ($)	Dec. 31, 2012 Ballantyne Re Plc and Orkney Re II Plc Below investment grade (BIG) Life Insurance [Member] Transaction	Mar. 31, 2012 Bank of America Agreement USD ($)	Dec. 31, 2011 Bank of America Agreement USD ($)	Dec. 31, 2012 Bank of America Agreement USD ($)	Apr. 14, 2011 Bank of America Agreement USD ($) Transaction	Apr. 14, 2011 Bank of America Agreement Second lien Transaction
Guarantor Obligations [Line Items]
Net Par Outstanding Financial Guaranty Insurance							$ 3,000,000,000	$ 1,900,000,000	$ 1,100,000,000	$ 5,700,000,000	$ 479,000,000	$ 206,000,000	$ 273,000,000											$ 297,000,000				$ 217,000,000	$ 327,000,000	$ 2,900,000,000	$ 2,800,000,000		$ 204,000,000
Percent of net par outstanding financial guaranty insurance in credit default swap form										72.00%
Guaranty Liabilities													32,000,000
Projected Net Expected Loss							54,000,000			27,000,000	197,000,000							7,000,000	16,000,000			35,000,000									139,000,000
Exposure of Debt																					314,000,000		455,000,000
Net exposure of debt											708,000,000			158,000,000		336,000,000					218,000,000		330,000,000
Claims Paid, Net														9,000,000						289,000,000
Liability for Unpaid Claims and Claims Adjustment Expense Net Largest Single Loss							25,000,000																	33,000,000	24,000,000
Number of Infrastructure Transactions			3
Financial Insurance Guaranty, Infrastructure Finance, Possible Claims Requiring Payment			1,400,000,000
Estimated Years for Recoveries on Infrastructure Transactions				20 years	45 years
Increase (Decrease) in Projected Expected Loss to be Paid							21,000,000			37,000,000												35,000,000
Loss and loss adjustment expense reserve	601,000,000	679,000,000													154,000,000		26,000,000														923,000,000	92,000,000
Net Claims to Date															13,000,000		88,000,000
Number of transactions insured																																		2				29
Maximum Loss Up To Which Loss Sharing Percentage Applicable																																						6,600,000,000
Collateral Losses																																					3,100,000,000
Insured Financial Obligations Gross Outstanding Principal Amount	552,039,000,000	614,342,000,000																																			3,500,000,000
Insured Financial Obligations Outstanding Principal Amount	519,893,000,000	558,048,000,000																								23,392,000,000	26,762,000,000										3,200,000,000
Financial Guaranty Insurance Contracts Recoveries for Breached Representations and Warranties																																			57,000,000	1,043,000,000
Financial Guaranty Insurance Contracts Total Recoveries Under Negotiated Agreement																																						1,100,000,000
Loss Sharing Percentage																																						80.00%
Number of transactions insured executed as financial guarantees																																							8
Judgment granted in favor of AGM						$ 90,000,000