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, 2020	Dec. 31, 2019
Credit Derivatives
Net Par Outstanding	$ 5,806	$ 5,956
BIG [Member]
Credit Derivatives
Net Par Outstanding	126	129
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member]
Credit Derivatives
Net Par Outstanding	$ 5,806	$ 5,956
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	$ 1,631	$ 1,730
Percentage of installment premiums denominated in currencies other than the U.S. dollar	28.10%	29.00%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, AA [Member]
Credit Derivatives
Net Par Outstanding	$ 1,702	$ 1,695
Percentage of installment premiums denominated in currencies other than the U.S. dollar	29.30%	28.50%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, A [Member]
Credit Derivatives
Net Par Outstanding	$ 1,127	$ 1,110
Percentage of installment premiums denominated in currencies other than the U.S. dollar	19.40%	18.60%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, BBB [Member]
Credit Derivatives
Net Par Outstanding	$ 1,220	$ 1,292
Percentage of installment premiums denominated in currencies other than the U.S. dollar	21.00%	21.70%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | BIG [Member]
Credit Derivatives
Net Par Outstanding	$ 126	$ 129
Percentage of installment premiums denominated in currencies other than the U.S. dollar	2.20%	2.20%