Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2020	Dec. 31, 2019
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (3)	$ 5
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 1,641	$ 1,306
Derivative Average Remaining Maturity	[1],[3]	3 years 10 months 24 days	3 years 8 months 12 days
Standard Poors AAA To A Minus Ratings [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 1	$ 2
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 127	$ 142
Derivative Average Remaining Maturity	[1],[3]	1 year 4 months 24 days	1 year 8 months 12 days
Standard Poors AAA To A Minus Ratings [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 1	$ 2
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 127	$ 142
Derivative Average Remaining Maturity	[1],[3]	1 year 4 months 24 days	1 year 8 months 12 days
Standard Poors BBB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 0	$ 3
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 281	$ 291
Derivative Average Remaining Maturity	[1],[3]	1 year 9 months 18 days	1 year 10 months 24 days
Standard Poors BBB Rating [Member] | Credit Default Swaps Referencing Indices [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (4)	$ 0
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 1,228	$ 873
Derivative Average Remaining Maturity	[1],[3]	4 years 8 months 12 days	4 years 8 months 12 days
Standard Poors BBB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ (4)	$ 3
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 1,509	$ 1,164
Derivative Average Remaining Maturity	[1],[3]	4 years 1 month 6 days	4 years
Standard & Poor's, BB Rating [Member]
Credit Derivatives [Line Items]
Derivative Average Remaining Maturity	[1],[2]	2 months 12 days	0 years
Standard & Poor's, BB Rating [Member] | Single Name Credit Default Swaps [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	$ 0	$ 0
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	5	0
Standard & Poor's, BB Rating [Member] | Credit Default Subtotal [Member]
Credit Derivatives [Line Items]
Credit Derivative Current Fair Value	[1]	0	0
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 5	$ 0

[1] The rating agency designations are based on ratings from Standard and Poor’s (“S&P”).

[2] Assumes the value of the referenced credit obligations is zero.

[3] The weighted average years to maturity of the credit default swaps is calculated based on weighted average notional amounts.