Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 208	$ 271
Maximum Amount of Future Payments under Credit Default Swaps	$ 11,421	$ 11,375
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	$ 47	$ 51
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,648	$ 2,643
Weighted Average Years to Maturity	2 years 6 months	2 years 8 months
Aaa/Aa/A | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 7	$ 7
Maximum Amount of Future Payments under Credit Default Swaps	$ 381	$ 375
Weighted Average Years to Maturity	2 years 5 months	2 years 7 months
Aaa/Aa/A | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 40	$ 44
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,267	$ 2,268
Weighted Average Years to Maturity	2 years 6 months	2 years 8 months
Baa
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 139	$ 190
Maximum Amount of Future Payments under Credit Default Swaps	$ 8,403	$ 8,267
Weighted Average Years to Maturity	5 years	4 years 10 months
Baa | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 7	$ 7
Maximum Amount of Future Payments under Credit Default Swaps	$ 656	$ 605
Weighted Average Years to Maturity	1 year 11 months	1 year 10 months
Baa | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 132	$ 183
Maximum Amount of Future Payments under Credit Default Swaps	$ 7,747	$ 7,662
Weighted Average Years to Maturity	5 years 2 months	5 years
Ba
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 0	$ 1
Maximum Amount of Future Payments under Credit Default Swaps	$ 20	$ 115
Weighted Average Years to Maturity	1 year 2 months	3 years 5 months
Ba | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 0	$ 1
Maximum Amount of Future Payments under Credit Default Swaps	$ 20	$ 115
Weighted Average Years to Maturity	1 year 2 months	3 years 5 months
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	$ 22	$ 29
Maximum Amount of Future Payments under Credit Default Swaps	$ 350	$ 350
Weighted Average Years to Maturity	5 years 1 month	4 years 11 months
B | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 2	$ 2
Maximum Amount of Future Payments under Credit Default Swaps	$ 20	$ 20
Weighted Average Years to Maturity	3 years 2 months	3 years 6 months
B | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 20	$ 27
Maximum Amount of Future Payments under Credit Default Swaps	$ 330	$ 330
Weighted Average Years to Maturity	5 years 2 months	5 years