Business Changes, Risks, Uncertainties and Accounting Developments (Details) (USD $)	3 Months Ended	6 Months Ended
	Mar. 31, 2011	Jun. 30, 2011 Y M Transactions Loans
RMBS
Financial guarantee obligations
Number of transactions insured		29
Gross par outstanding of transactions covered by the Bank of America Agreement		$ 4,700,000,000
Net par outstanding of transactions covered by the Bank of America Agreement		4,400,000,000
Percentage of Assured Guaranty's total below-investment-grade ("BIG") RMBS net par outstanding covered by the Bank of America Agreement (as a percent)		28.00%
Total first lien
Financial guarantee obligations
Number of transactions insured		21
Net par outstanding of transactions covered by the Bank of America Agreement		1,900,000,000
Percentage of Assured Guaranty's total below-investment-grade ("BIG") RMBS net par outstanding covered by the Bank of America Agreement (as a percent)		17.00%
Loss sharing agreement (as a percent)		80.00%
Maximum aggregate collateral losses		6,600,000,000
Eligible assets in trust for payment of reimbursement		1,000,000,000
Increased estimate of expected R&W recoveries	411,200,000
Increase to pre-tax income	220,000,000
Second lien
Financial guarantee obligations
Number of transactions insured		8
Payment made by Bank of America		928,100,000
Bank of America's amount obligated to pay		171,900,000
Total consideration under Bank of America agreement		$ 1,100,000,000