Contracts Accounted for as Credit Derivatives - Distribution of Credit Derivative Net Par Outstanding by Internal Rating (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Credit Derivatives
Net Par Outstanding	$ 5,658	$ 5,956
BIG
Credit Derivatives
Net Par Outstanding	122	129
Credit Concentration Risk | Derivative, Aggregate Notional Amount
Credit Derivatives
Net Par Outstanding	$ 5,658	$ 5,956
Percentage of installment premiums denominated in currencies other than the U.S. dollar	100.00%	100.00%
Credit Concentration Risk | Derivative, Aggregate Notional Amount | AAA
Credit Derivatives
Net Par Outstanding	$ 1,613	$ 1,730
Percentage of installment premiums denominated in currencies other than the U.S. dollar	28.50%	29.00%
Credit Concentration Risk | Derivative, Aggregate Notional Amount | AA
Credit Derivatives
Net Par Outstanding	$ 1,699	$ 1,695
Percentage of installment premiums denominated in currencies other than the U.S. dollar	30.00%	28.50%
Credit Concentration Risk | Derivative, Aggregate Notional Amount | A
Credit Derivatives
Net Par Outstanding	$ 767	$ 1,110
Percentage of installment premiums denominated in currencies other than the U.S. dollar	13.50%	18.60%
Credit Concentration Risk | Derivative, Aggregate Notional Amount | BBB
Credit Derivatives
Net Par Outstanding	$ 1,457	$ 1,292
Percentage of installment premiums denominated in currencies other than the U.S. dollar	25.80%	21.70%
Credit Concentration Risk | Derivative, Aggregate Notional Amount | BIG
Credit Derivatives
Net Par Outstanding	$ 122	$ 129
Percentage of installment premiums denominated in currencies other than the U.S. dollar	2.20%	2.20%