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, 2020	Dec. 31, 2019
Credit Derivatives
Net Par Outstanding	$ 5,765	$ 5,956
BIG [Member]
Credit Derivatives
Net Par Outstanding	125	129
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member]
Credit Derivatives
Net Par Outstanding	$ 5,765	$ 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	$ 1,594	$ 1,730
Percentage of installment premiums denominated in currencies other than the U.S. dollar	27.60%	29.00%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, AA [Member]
Credit Derivatives
Net Par Outstanding	$ 1,728	$ 1,695
Percentage of installment premiums denominated in currencies other than the U.S. dollar	30.00%	28.50%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, A [Member]
Credit Derivatives
Net Par Outstanding	$ 848	$ 1,110
Percentage of installment premiums denominated in currencies other than the U.S. dollar	14.70%	18.60%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, BBB [Member]
Credit Derivatives
Net Par Outstanding	$ 1,470	$ 1,292
Percentage of installment premiums denominated in currencies other than the U.S. dollar	25.50%	21.70%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | BIG [Member]
Credit Derivatives
Net Par Outstanding	$ 125	$ 129
Percentage of installment premiums denominated in currencies other than the U.S. dollar	2.20%	2.20%