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, 2018	Dec. 31, 2017
Credit Derivatives
Net Par Outstanding	$ 6,681	$ 6,207
BIG [Member]
Credit Derivatives
Net Par Outstanding	253	338
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member]
Credit Derivatives
Net Par Outstanding	$ 6,681	$ 6,207
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	$ 2,025	$ 2,144
Percentage of installment premiums denominated in currencies other than the U.S. dollar	30.30%	34.60%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, AA [Member]
Credit Derivatives
Net Par Outstanding	$ 1,718	$ 1,170
Percentage of installment premiums denominated in currencies other than the U.S. dollar	25.70%	18.80%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, A [Member]
Credit Derivatives
Net Par Outstanding	$ 1,293	$ 1,517
Percentage of installment premiums denominated in currencies other than the U.S. dollar	19.40%	24.50%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, BBB [Member]
Credit Derivatives
Net Par Outstanding	$ 1,392	$ 1,038
Percentage of installment premiums denominated in currencies other than the U.S. dollar	20.80%	16.70%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | BIG [Member]
Credit Derivatives
Net Par Outstanding	$ 253	$ 338
Percentage of installment premiums denominated in currencies other than the U.S. dollar	3.80%	5.40%