Financial Guaranty Insurance Losses (Ratings Impact on Financial Guaranty Insurance Business) (Details 6) (USD $)	3 Months Ended
	Dec. 31, 2012
AGM
Guarantor Obligations [Line Items]
Maximum possible estimated termination payment on swap obligations after taking account the rating downgrade in January 2013	109,000,000
Maximum possible estimated termination payment on swap obligations after taking account the possibility of further rating downgrades	258,000,000
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	12,300,000,000
Financial Guaranty Insurance Contracts, Principal Amount BBB- or below	600,000,000
Financial guaranty insurance contracts, remarketed, estimated maximum increase to interest rates
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