Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 26	$ 36
Maximum Amount of Future Payments under Credit Default Swaps	$ 1,793	$ 1,613
Weighted Average Years to Maturity (2)	4 years	4 years 1 month 6 days
Credit default swaps | Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 8	$ 11
Maximum Amount of Future Payments under Credit Default Swaps	$ 879	$ 615
Weighted Average Years to Maturity (2)	2 years 6 months	2 years 6 months
Credit default swaps | Baa
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 18	$ 25
Maximum Amount of Future Payments under Credit Default Swaps	$ 914	$ 998
Weighted Average Years to Maturity (2)	5 years 6 months	5 years 1 month 6 days