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, 2021	Dec. 31, 2020
Credit Derivatives
Net Par Outstanding	$ 4,286	$ 5,371
BIG
Credit Derivatives
Net Par Outstanding	62	120
Credit Concentration Risk | Derivative, Aggregate Notional Amount
Credit Derivatives
Net Par Outstanding	$ 4,286	$ 5,371
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	$ 1,513	$ 1,796
Percentage of installment premiums denominated in currencies other than the U.S. dollar	35.30%	33.50%
Credit Concentration Risk | Derivative, Aggregate Notional Amount | AA
Credit Derivatives
Net Par Outstanding	$ 1,323	$ 1,541
Percentage of installment premiums denominated in currencies other than the U.S. dollar	30.90%	28.70%
Credit Concentration Risk | Derivative, Aggregate Notional Amount | A
Credit Derivatives
Net Par Outstanding	$ 544	$ 758
Percentage of installment premiums denominated in currencies other than the U.S. dollar	12.70%	14.10%
Credit Concentration Risk | Derivative, Aggregate Notional Amount | BBB
Credit Derivatives
Net Par Outstanding	$ 844	$ 1,156
Percentage of installment premiums denominated in currencies other than the U.S. dollar	19.70%	21.50%
Credit Concentration Risk | Derivative, Aggregate Notional Amount | BIG
Credit Derivatives
Net Par Outstanding	$ 62	$ 120
Percentage of installment premiums denominated in currencies other than the U.S. dollar	1.40%	2.20%