Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 195	$ 82
Maximum Amount of Future Payments under Credit Default Swaps	$ 11,409	$ 11,391
Weighted Average Years to Maturity	4 years 5 months	4 years 4 months
Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 38	$ 32
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,708	$ 2,508
Weighted Average Years to Maturity	2 years 4 months	2 years 5 months
Aaa/Aa/A | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 5	$ 4
Maximum Amount of Future Payments under Credit Default Swaps	$ 391	$ 354
Weighted Average Years to Maturity	1 year 6 months	1 year 8 months
Aaa/Aa/A | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 33	$ 28
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,317	$ 2,154
Weighted Average Years to Maturity	2 years 5 months	2 years 6 months
Baa
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 136	$ 43
Maximum Amount of Future Payments under Credit Default Swaps	$ 8,361	$ 8,538
Weighted Average Years to Maturity	5 years 1 month	4 years 10 months
Baa | 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	$ 404	$ 482
Weighted Average Years to Maturity	1 year 5 months	1 year 6 months
Baa | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 133	$ 40
Maximum Amount of Future Payments under Credit Default Swaps	$ 7,957	$ 8,056
Weighted Average Years to Maturity	5 years 4 months	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	$ 10	$ 15
Weighted Average Years to Maturity	1 year 2 months	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	$ 10	$ 15
Weighted Average Years to Maturity	1 year 2 months	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	$ 21	$ 7
Maximum Amount of Future Payments under Credit Default Swaps	$ 330	$ 330
Weighted Average Years to Maturity	5 years 2 months	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	$ 0	$ 0
Weighted Average Years to Maturity	0 years	0 years
B | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 21	$ 7
Maximum Amount of Future Payments under Credit Default Swaps	$ 330	$ 330
Weighted Average Years to Maturity	5 years 2 months	5 years