Business Changes, Risks, Uncertainties and Accounting Developments (Details) (USD $)	9 Months Ended
	Sep. 30, 2011 transaction
RMBS
Financial guarantee obligations
Number of transactions insured	29
Gross par outstanding of transactions covered by the Bank of America Agreement	$ 4,600,000,000
Net par outstanding of transactions covered by the Bank of America Agreement	4,200,000,000
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
Loss sharing agreement (as a percent)	80.00%
Maximum aggregate collateral losses	6,600,000,000
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%
Eligible assets in trust for payment of reimbursement	965,000,000
Estimate of expected R&W recoveries	615,100,000
Second lien
Financial guarantee obligations
Number of transactions insured	8
Payment made by Bank of America	985,400,000
Bank of America's amount obligated to pay	114,600,000
Total consideration under Bank of America agreement	$ 1,100,000,000