Financial Guaranty Contracts Accounted for as Credit Derivatives - Distribution of Credit Derivative Net Par Outstanding by Internal Rating (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Credit Derivatives
Net Par Outstanding	34,996	54,482
Internal Credit Rating, AAA [Member]
Credit Derivatives
Net Par Outstanding	21,817	38,244
Internal Credit Rating, AA [Member]
Credit Derivatives
Net Par Outstanding	5,398	3,648
Internal Credit Rating, A [Member]
Credit Derivatives
Net Par Outstanding	1,982	3,636
Internal Credit Rating, BBB [Member]
Credit Derivatives
Net Par Outstanding	2,774	4,161
BIG [Member]
Credit Derivatives
Net Par Outstanding	3,025	4,793
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member]
Credit Derivatives
Percentage of installment premiums denominated in currencies other than the U.S. dollar	100.00%	100.00%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, AAA [Member]
Credit Derivatives
Percentage of installment premiums denominated in currencies other than the U.S. dollar	62.30%	70.20%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, AA [Member]
Credit Derivatives
Percentage of installment premiums denominated in currencies other than the U.S. dollar	15.40%	6.70%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, A [Member]
Credit Derivatives
Percentage of installment premiums denominated in currencies other than the U.S. dollar	5.70%	6.70%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, BBB [Member]
Credit Derivatives
Percentage of installment premiums denominated in currencies other than the U.S. dollar	8.00%	7.60%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | BIG [Member]
Credit Derivatives
Percentage of installment premiums denominated in currencies other than the U.S. dollar	8.60%	8.80%