Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 30	$ 11
Maximum Amount of Future Payments under Credit Default Swaps	$ 1,712	$ 1,798
Weighted Average Years to Maturity (2)	4 years 3 months 18 days	3 years 10 months 24 days
Credit default swaps | Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 9	$ 8
Maximum Amount of Future Payments under Credit Default Swaps	$ 615	$ 689
Weighted Average Years to Maturity (2)	2 years 4 months 24 days	2 years
Credit default swaps | Baa
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 21	$ 3
Maximum Amount of Future Payments under Credit Default Swaps	$ 1,097	$ 1,109
Weighted Average Years to Maturity (2)	5 years 3 months 18 days	5 years