Business Changes, Risks, Uncertainties and Accounting Developments (Details 2) (USD $)	Sep. 30, 2012	May 08, 2012
Financial guarantee obligations
Estimated Withdrawal of Guaranteed Investment Contracts Funds That Would Result From a Downgrade to Assured Guaranty Municipal Corp	$ 358,000,000
Required Collateral on Guaranteed Investment Contracts in the Event of a Downgrade to Assured Guaranty Municipal Corp	2,900,000,000
Assumed Average Margin of Collateral Percentage Under the Event of a Downgrade to Assured Guaranty Municipal Corp	105.00%
Market Value of Guaranteed Investment Contracts Necessary to Avoid Withdrawal of Funds in the Event of a Downgrade Assuming an Average Margin of 105 Percent	3,100,000,000
Cash payment to be received		166,000,000
Eligible assets to be placed in trust to collateralize the obligations of a reinsurance affiliate		282,000,000
Minimum | Cash
Financial guarantee obligations
Collateralization Percentage Related to Guaranteed Investment Conracts Insured by Assured Guaranty Municipal Corp	100.00%
Maximum | Asset-backed securities
Financial guarantee obligations
Collateralization Percentage Related to Guaranteed Investment Conracts Insured by Assured Guaranty Municipal Corp	108.00%
Covered Transactions
Financial guarantee obligations
Number of transactions insured		8
Loss sharing percentage, first layer		80.00%
Maximum aggregate collateral losses up to which first layer of loss sharing will be applicatble		319,000,000
Minimum losses at which second later of loss sharing percentage becomes applicable		389,000,000
Maximum aggregate collateral losses up to which second layer of loss sharing will be applicable		600,000,000
Loss sharing percentage, second layer		85.00%
Gross Par Outstanding	548,000,000
Insured Financial Obligations Net Outstanding Principal Amount	472,000,000
Uninsured Tranches
Financial guarantee obligations
Loss sharing percentage, first layer		60.00%
Maximum aggregate collateral losses up to which first layer of loss sharing will be applicatble		141,000,000
Minimum losses at which second later of loss sharing percentage becomes applicable		161,000,000
Loss sharing percentage, second layer		60.00%
Gross Par Outstanding	317,000,000
Minimum losses at which third layer of loss sharing percentage becomes applicable		185,000,000
Loss sharing percentage, third layer		100.00%
Maximum aggregate collateral losses up to which third layer of loss sharing will be applicable		248,000,000
Losses Incurred by the Company	$ 141,000,000
RMBS
Financial guarantee obligations
Number of transactions, against which litigations have been settled		3