Financial Guaranty Contracts Accounted for as Credit Derivatives - Distribution of Credit Derivative Net Par Outstanding by Internal Rating (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Credit Derivatives
Net Par Outstanding	$ 24,866	$ 25,594
Internal Credit Rating, AAA [Member]
Credit Derivatives
Net Par Outstanding	14,302	14,808
Internal Credit Rating, AA [Member]
Credit Derivatives
Net Par Outstanding	4,624	4,821
Internal Credit Rating, A [Member]
Credit Derivatives
Net Par Outstanding	2,253	2,144
Internal Credit Rating, BBB [Member]
Credit Derivatives
Net Par Outstanding	2,108	2,212
BIG [Member]
Credit Derivatives
Net Par Outstanding	$ 1,579	$ 1,609
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	57.50%	57.90%
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	18.60%	18.80%
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	9.10%	8.40%
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	8.50%	8.60%
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	6.30%	6.30%