Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2021	Sep. 30, 2020	Dec. 31, 2020
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 178		$ 196
Maximum Amount of Future Payments under Credit Default Swaps	$ 9,358		9,609
Weighted Average Years to Maturity	4 years 8 months 12 days	4 years 9 months 18 days
Credit Risk Derivative Assets, at Fair Value	$ 178		196
Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	28		32
Maximum Amount of Future Payments under Credit Default Swaps	$ 1,982		1,987
Weighted Average Years to Maturity	2 years 2 months 12 days	2 years 6 months
Aaa/Aa/A | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 4		5
Maximum Amount of Future Payments under Credit Default Swaps	$ 203		208
Weighted Average Years to Maturity	2 years 4 months 24 days	2 years 8 months 12 days
Aaa/Aa/A | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 24		27
Maximum Amount of Future Payments under Credit Default Swaps	$ 1,779		1,779
Weighted Average Years to Maturity	2 years 2 months 12 days	2 years 6 months
Baa [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 145		159
Maximum Amount of Future Payments under Credit Default Swaps	$ 7,321		7,567
Weighted Average Years to Maturity	5 years 7 months 6 days	5 years 4 months 24 days
Baa [Member] | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 3		3
Maximum Amount of Future Payments under Credit Default Swaps	$ 244		249
Weighted Average Years to Maturity	2 years 2 months 12 days	2 years 6 months
Baa [Member] | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 142		156
Maximum Amount of Future Payments under Credit Default Swaps	$ 7,077		7,318
Weighted Average Years to Maturity	5 years 7 months 6 days	5 years 6 months
B
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 5		5
Maximum Amount of Future Payments under Credit Default Swaps	$ 55		55
Weighted Average Years to Maturity	4 years 8 months 12 days	5 years
B | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 5		5
Maximum Amount of Future Payments under Credit Default Swaps	$ 55		$ 55
Weighted Average Years to Maturity	4 years 8 months 12 days	5 years