Contracts Accounted for as Credit Derivatives - Distribution of Credit Derivative Net Par Outstanding by Internal Rating (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Credit Derivatives
Net Par Outstanding	$ 6,120	$ 6,182
BIG [Member]
Credit Derivatives
Net Par Outstanding	153	157
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member]
Credit Derivatives
Net Par Outstanding	$ 6,120	$ 6,182
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	$ 1,795	$ 1,813
Percentage of installment premiums denominated in currencies other than the U.S. dollar	29.30%	29.40%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, AA [Member]
Credit Derivatives
Net Par Outstanding	$ 1,665	$ 1,690
Percentage of installment premiums denominated in currencies other than the U.S. dollar	27.20%	27.30%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, A [Member]
Credit Derivatives
Net Par Outstanding	$ 1,180	$ 1,171
Percentage of installment premiums denominated in currencies other than the U.S. dollar	19.30%	18.90%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, BBB [Member]
Credit Derivatives
Net Par Outstanding	$ 1,327	$ 1,351
Percentage of installment premiums denominated in currencies other than the U.S. dollar	21.70%	21.90%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | BIG [Member]
Credit Derivatives
Net Par Outstanding	$ 153	$ 157
Percentage of installment premiums denominated in currencies other than the U.S. dollar	2.50%	2.50%