Financial Guaranty Contracts Accounted for as Credit Derivatives - Distribution of Credit Derivative Net Par Outstanding by Internal Rating (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Credit Derivatives
Net Par Outstanding	$ 20,014	$ 25,594
BIG [Member]
Credit Derivatives
Net Par Outstanding	1,086	1,609
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member]
Credit Derivatives
Net Par Outstanding	$ 20,014	$ 25,594
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
Net Par Outstanding	$ 10,966	$ 14,808
Percentage of installment premiums denominated in currencies other than the U.S. dollar	54.80%	57.90%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, AA [Member]
Credit Derivatives
Net Par Outstanding	$ 4,137	$ 4,821
Percentage of installment premiums denominated in currencies other than the U.S. dollar	20.70%	18.80%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, A [Member]
Credit Derivatives
Net Par Outstanding	$ 2,067	$ 2,144
Percentage of installment premiums denominated in currencies other than the U.S. dollar	10.30%	8.40%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, BBB [Member]
Credit Derivatives
Net Par Outstanding	$ 1,758	$ 2,212
Percentage of installment premiums denominated in currencies other than the U.S. dollar	8.80%	8.60%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | BIG [Member]
Credit Derivatives
Net Par Outstanding	$ 1,086	$ 1,609
Percentage of installment premiums denominated in currencies other than the U.S. dollar	5.40%	6.30%