Contracts Accounted for as Credit Derivatives - Distribution of Credit Derivative Net Par Outstanding by Internal Rating (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Credit Derivatives
Net Par Outstanding	$ 7,535	$ 16,997
BIG [Member]
Credit Derivatives
Net Par Outstanding	696	973
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member]
Credit Derivatives
Net Par Outstanding	$ 7,535	$ 16,997
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	$ 2,956	$ 10,967
Percentage of installment premiums denominated in currencies other than the U.S. dollar	39.30%	64.60%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, AA [Member]
Credit Derivatives
Net Par Outstanding	$ 1,247	$ 2,167
Percentage of installment premiums denominated in currencies other than the U.S. dollar	16.50%	12.70%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, A [Member]
Credit Derivatives
Net Par Outstanding	$ 1,628	$ 1,499
Percentage of installment premiums denominated in currencies other than the U.S. dollar	21.60%	8.80%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, BBB [Member]
Credit Derivatives
Net Par Outstanding	$ 1,008	$ 1,391
Percentage of installment premiums denominated in currencies other than the U.S. dollar	13.40%	8.20%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | BIG [Member]
Credit Derivatives
Net Par Outstanding	$ 696	$ 973
Percentage of installment premiums denominated in currencies other than the U.S. dollar	9.20%	5.70%