Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 32	$ 11
Maximum Amount of Future Payments under Credit Default Swaps	$ 1,883	$ 1,798
Weighted Average Years to Maturity (2)	3 years 8 months 12 days	3 years 10 months 24 days
Credit default swaps | Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 10	$ 8
Maximum Amount of Future Payments under Credit Default Swaps	$ 828	$ 689
Weighted Average Years to Maturity (2)	2 years	2 years
Credit default swaps | Baa
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 22	$ 3
Maximum Amount of Future Payments under Credit Default Swaps	$ 1,055	$ 1,109
Weighted Average Years to Maturity (2)	4 years 10 months 24 days	5 years