Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ (60)	$ 271
Maximum Amount of Future Payments under Credit Default Swaps	$ 11,353	$ 11,520
Weighted Average Years to Maturity	4 years 4 months 24 days	4 years 3 months 18 days
Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 3	$ 39
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,500	$ 2,473
Weighted Average Years to Maturity	2 years 1 month 6 days	2 years 2 months 12 days
Aaa/Aa/A | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 3	$ 4
Maximum Amount of Future Payments under Credit Default Swaps	$ 241	$ 298
Weighted Average Years to Maturity	2 years	1 year 8 months 12 days
Aaa/Aa/A | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 0	$ 35
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,259	$ 2,175
Weighted Average Years to Maturity	2 years 1 month 6 days	2 years 2 months 12 days
Baa [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ (43)	$ 206
Maximum Amount of Future Payments under Credit Default Swaps	$ 8,534	$ 8,755
Weighted Average Years to Maturity	5 years 1 month 6 days	4 years 10 months 24 days
Baa [Member] | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ (1)	$ 3
Maximum Amount of Future Payments under Credit Default Swaps	$ 289	$ 216
Weighted Average Years to Maturity	1 year 10 months 24 days	1 year 6 months
Baa [Member] | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ (42)	$ 203
Maximum Amount of Future Payments under Credit Default Swaps	$ 8,245	$ 8,539
Weighted Average Years to Maturity	5 years 2 months 12 days	5 years
Ba
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ (2)	$ 0
Maximum Amount of Future Payments under Credit Default Swaps	$ 24	$ 9
Weighted Average Years to Maturity	2 years 8 months 12 days	5 years
Ba | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ (2)	$ 0
Maximum Amount of Future Payments under Credit Default Swaps	$ 24	$ 9
Weighted Average Years to Maturity	2 years 8 months 12 days	5 years
Ba | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 0	$ 0
Maximum Amount of Future Payments under Credit Default Swaps	$ 0	$ 0
Weighted Average Years to Maturity	0 years	0 years
B
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ (18)	$ 26
Maximum Amount of Future Payments under Credit Default Swaps	$ 295	$ 283
Weighted Average Years to Maturity	4 years 8 months 12 days	4 years 9 months 18 days
B | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 0	$ 0
Maximum Amount of Future Payments under Credit Default Swaps	$ 24	$ 10
Weighted Average Years to Maturity	2 years 10 months 24 days	6 months
B | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ (18)	$ 26
Maximum Amount of Future Payments under Credit Default Swaps	$ 271	$ 273
Weighted Average Years to Maturity	4 years 9 months 18 days	5 years