Financial Guaranty Insurance Losses (Ratings Impact on Financial Guaranty Insurance Business) (Details 7) (USD $)	3 Months Ended
	Mar. 31, 2013
AGM
Guarantor Obligations [Line Items]
Maximum possible estimated termination payment on swap obligations after taking account the rating downgrade in January 2013	132,000,000
Maximum possible estimated termination payment on swap obligations after taking account the possibility of further rating downgrades	236,000,000
Variable rate demand obligations
Guarantor Obligations [Line Items]
Rate basis for bank bond rate	prime rate
Variable rate demand obligations | AGM and AGC
Guarantor Obligations [Line Items]
Financial guaranty insurance contracts principal amount insured	8,700,000,000
Financial Guaranty Insurance Contracts, Principal Amount BBB- or below	500,000,000
Financial guaranty insurance contracts, remarketed, estimated maximum increase to interest rates
Minimum
Guarantor Obligations [Line Items]
GIC Terminated as a Result of Downgrade	400,000,000
Minimum | Variable rate demand obligations
Guarantor Obligations [Line Items]
Bank bond rate (as a percent)	2.00%
Threshold period of bonds held by bank for right of accelerated repayment (in days)	90 days
Maximum | Variable rate demand obligations
Guarantor Obligations [Line Items]
Bank bond rate (as a percent)	3.00%
Bank bond capped rate (as a percent)	25.00%
Threshold period of bonds held by bank for right of accelerated repayment (in days)	180 days
Cash | Minimum
Guarantor Obligations [Line Items]
Collaterlization as a Percentage of GIC Balance	100.00%
Asset-backed securities | Maximum
Guarantor Obligations [Line Items]
Collaterlization as a Percentage of GIC Balance	108.00%