Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) - USD ($) $ in Millions		9 Months Ended		12 Months Ended
		Sep. 30, 2020		Dec. 31, 2019
Credit Derivatives [Line Items]
Document Period End Date		Sep. 30, 2020
Credit Derivative Current Fair Value	[1]	$ 3		$ 5
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 1,307		$ 1,306
Derivative Average Remaining Maturity	[1],[3]	3 years 7 months 6 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]	$ 87		$ 142
Derivative Average Remaining Maturity	[1],[3]	1 year 6 months		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]	$ 87		$ 142
Derivative Average Remaining Maturity	[1],[3]	1 year 6 months		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]	$ 2		$ 3
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 222		$ 291
Derivative Average Remaining Maturity	[1],[3]	1 year 8 months 12 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]	$ 0		$ 0
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 988		$ 873
Derivative Average Remaining Maturity	[1],[3]	4 years 2 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]	$ 2		$ 3
Maximum Potential Future Exposure on Credit Risk Derivatives	[1],[2]	$ 1,210		$ 1,164
Derivative Average Remaining Maturity	[1],[3]	3 years 8 months 12 days		4 years
Standard & Poor's, BB Rating [Member]
Credit Derivatives [Line Items]
Derivative Average Remaining Maturity	[1]	1 year	[3]	0 years	[2]
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]	10		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]	$ 10		$ 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.