Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 11	$ 40
Maximum Amount of Future Payments under Credit Default Swaps	$ 1,798	$ 1,878
Weighted Average Years to Maturity	3 years 11 months	4 years 1 month
Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 8	$ 12
Maximum Amount of Future Payments under Credit Default Swaps	$ 689	$ 558
Weighted Average Years to Maturity	1 year 12 months	2 years 9 months
Baa
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 3	$ 28
Maximum Amount of Future Payments under Credit Default Swaps	$ 1,109	$ 1,295
Weighted Average Years to Maturity	4 years 12 months	4 years 8 months
Ba
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 0	$ 0
Maximum Amount of Future Payments under Credit Default Swaps	$ 0	$ 25
Weighted Average Years to Maturity	0 years	4 years 6 months