Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ (6)	$ 36
Maximum Amount of Future Payments under Credit Default Swaps	$ 1,876	$ 1,613
Weighted Average Years to Maturity (2)	4 years 4 months 24 days	4 years 1 month 6 days
Credit default swaps | Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 3	$ 11
Maximum Amount of Future Payments under Credit Default Swaps	$ 680	$ 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	$ (9)	$ 25
Maximum Amount of Future Payments under Credit Default Swaps	$ 1,196	$ 998
Weighted Average Years to Maturity (2)	5 years 4 months 24 days	5 years 1 month 6 days