Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 255	$ 152
Maximum Amount of Future Payments under Credit Default Swaps	$ 11,946	$ 10,732
Weighted Average Years to Maturity	4 years 6 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	$ 2,713	$ 2,784
Weighted Average Years to Maturity	2 years 11 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	$ 8	$ 6
Maximum Amount of Future Payments under Credit Default Swaps	$ 445	$ 449
Weighted Average Years to Maturity	2 years 6 months	3 years 1 month
Aaa/Aa/A | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 43	$ 34
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,268	$ 2,335
Weighted Average Years to Maturity	3 years	3 years 7 months
Baa
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 176	$ 93
Maximum Amount of Future Payments under Credit Default Swaps	$ 8,758	$ 7,462
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	$ 8	$ 5
Maximum Amount of Future Payments under Credit Default Swaps	$ 685	$ 751
Weighted Average Years to Maturity	1 year 11 months	2 years 6 months
Baa | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 168	$ 88
Maximum Amount of Future Payments under Credit Default Swaps	$ 8,073	$ 6,711
Weighted Average Years to Maturity	5 years 4 months	5 years
Ba
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 0	$ (2)
Maximum Amount of Future Payments under Credit Default Swaps	$ 115	$ 135
Weighted Average Years to Maturity	3 years 7 months	4 years 1 month
Ba | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 0	$ (2)
Maximum Amount of Future Payments under Credit Default Swaps	$ 115	$ 135
Weighted Average Years to Maturity	3 years 7 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
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 28	$ 21
Maximum Amount of Future Payments under Credit Default Swaps	$ 360	$ 351
Weighted Average Years to Maturity	5 years	4 years 4 months
B | 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	$ 30	$ 70
Weighted Average Years to Maturity	2 years 7 months	1 year 10 months
B | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 26	$ 20
Maximum Amount of Future Payments under Credit Default Swaps	$ 330	$ 281
Weighted Average Years to Maturity	5 years 2 months	5 years