Contracts Accounted for as Credit Derivatives - Distribution of Credit Derivative Net Par Outstanding by Internal Rating (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Credit Derivatives
Net Par Outstanding	$ 6,207	$ 16,997
BIG [Member]
Credit Derivatives
Net Par Outstanding	338	973
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member]
Credit Derivatives
Net Par Outstanding	$ 6,207	$ 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	$ 2,144	$ 10,967
Percentage of installment premiums denominated in currencies other than the U.S. dollar	34.60%	64.60%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, AA [Member]
Credit Derivatives
Net Par Outstanding	$ 1,170	$ 2,167
Percentage of installment premiums denominated in currencies other than the U.S. dollar	18.80%	12.70%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, A [Member]
Credit Derivatives
Net Par Outstanding	$ 1,517	$ 1,499
Percentage of installment premiums denominated in currencies other than the U.S. dollar	24.50%	8.80%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, BBB [Member]
Credit Derivatives
Net Par Outstanding	$ 1,038	$ 1,391
Percentage of installment premiums denominated in currencies other than the U.S. dollar	16.70%	8.20%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | BIG [Member]
Credit Derivatives
Net Par Outstanding	$ 338	$ 973
Percentage of installment premiums denominated in currencies other than the U.S. dollar	5.40%	5.70%