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, 2018	Dec. 31, 2017
Credit Derivatives
Net Par Outstanding	$ 6,182	$ 6,207
BIG [Member]
Credit Derivatives
Net Par Outstanding	157	338
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member]
Credit Derivatives
Net Par Outstanding	$ 6,182	$ 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	$ 1,813	$ 2,144
Percentage of installment premiums denominated in currencies other than the U.S. dollar	29.40%	34.60%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, AA [Member]
Credit Derivatives
Net Par Outstanding	$ 1,690	$ 1,170
Percentage of installment premiums denominated in currencies other than the U.S. dollar	27.30%	18.80%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, A [Member]
Credit Derivatives
Net Par Outstanding	$ 1,171	$ 1,517
Percentage of installment premiums denominated in currencies other than the U.S. dollar	18.90%	24.50%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, BBB [Member]
Credit Derivatives
Net Par Outstanding	$ 1,351	$ 1,038
Percentage of installment premiums denominated in currencies other than the U.S. dollar	21.90%	16.70%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | BIG [Member]
Credit Derivatives
Net Par Outstanding	$ 157	$ 338
Percentage of installment premiums denominated in currencies other than the U.S. dollar	2.50%	5.40%