Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 271	$ 152
Maximum Amount of Future Payments under Credit Default Swaps	$ 11,375	$ 10,732
Weighted Average Years to Maturity	4 years 4 months	4 years 5 months
Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 51	$ 40
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,643	$ 2,784
Weighted Average Years to Maturity	2 years 8 months	3 years 6 months
Aaa/Aa/A | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 7	$ 6
Maximum Amount of Future Payments under Credit Default Swaps	$ 375	$ 449
Weighted Average Years to Maturity	2 years 7 months	3 years 1 month
Aaa/Aa/A | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 44	$ 34
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,268	$ 2,335
Weighted Average Years to Maturity	2 years 8 months	3 years 7 months
Baa
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 190	$ 93
Maximum Amount of Future Payments under Credit Default Swaps	$ 8,267	$ 7,462
Weighted Average Years to Maturity	4 years 10 months	4 years 10 months
Baa | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 7	$ 5
Maximum Amount of Future Payments under Credit Default Swaps	$ 605	$ 751
Weighted Average Years to Maturity	1 year 10 months	2 years 6 months
Baa | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 183	$ 88
Maximum Amount of Future Payments under Credit Default Swaps	$ 7,662	$ 6,711
Weighted Average Years to Maturity	5 years	5 years
Ba
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 1	$ (2)
Maximum Amount of Future Payments under Credit Default Swaps	$ 115	$ 135
Weighted Average Years to Maturity	3 years 5 months	4 years 1 month
Ba | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 1	$ (2)
Maximum Amount of Future Payments under Credit Default Swaps	$ 115	$ 135
Weighted Average Years to Maturity	3 years 5 months	4 years 1 month
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 [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 29	$ 21
Maximum Amount of Future Payments under Credit Default Swaps	$ 350	$ 351
Weighted Average Years to Maturity	4 years 11 months	4 years 4 months
B [Member] | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 2	$ 1
Maximum Amount of Future Payments under Credit Default Swaps	$ 20	$ 70
Weighted Average Years to Maturity	3 years 6 months	1 year 10 months
B [Member] | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 27	$ 20
Maximum Amount of Future Payments under Credit Default Swaps	$ 330	$ 281
Weighted Average Years to Maturity	5 years	5 years