Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 229	$ 82
Maximum Amount of Future Payments under Credit Default Swaps	$ 11,583	$ 11,391
Weighted Average Years to Maturity	4 years 6 months	4 years 3 months 18 days
Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 39	$ 32
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,514	$ 2,508
Weighted Average Years to Maturity	2 years 3 months 18 days	2 years 4 months 24 days
Aaa/Aa/A | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 4	$ 4
Maximum Amount of Future Payments under Credit Default Swaps	$ 315	$ 354
Weighted Average Years to Maturity	1 year 7 months 6 days	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	$ 35	$ 28
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,199	$ 2,154
Weighted Average Years to Maturity	2 years 4 months 24 days	2 years 6 months
Baa [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 170	$ 43
Maximum Amount of Future Payments under Credit Default Swaps	$ 8,770	$ 8,538
Weighted Average Years to Maturity	5 years 2 months 12 days	4 years 9 months 18 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	$ 266	$ 482
Weighted Average Years to Maturity	1 year 8 months 12 days	1 year 6 months
Baa [Member] | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 167	$ 40
Maximum Amount of Future Payments under Credit Default Swaps	$ 8,504	$ 8,056
Weighted Average Years to Maturity	5 years 3 months 18 days	5 years
Ba
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 0	$ 0
Maximum Amount of Future Payments under Credit Default Swaps	$ 0	$ 15
Weighted Average Years to Maturity	0 years	2 years
Ba | 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	$ 0	$ 15
Weighted Average Years to Maturity	0 years	2 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	$ 20	$ 7
Maximum Amount of Future Payments under Credit Default Swaps	$ 299	$ 330
Weighted Average Years to Maturity	4 years 7 months 6 days	5 years
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	$ 10	$ 0
Weighted Average Years to Maturity	8 months 12 days	0 years
B | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 20	$ 7
Maximum Amount of Future Payments under Credit Default Swaps	$ 289	$ 330
Weighted Average Years to Maturity	4 years 8 months 12 days	5 years