Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 255	$ 180
Maximum Amount of Future Payments under Credit Default Swaps	$ 13,377	$ 12,645
Weighted Average Years to Maturity	4 years 5 months	4 years 5 months
Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 60	$ 48
Maximum Amount of Future Payments under Credit Default Swaps	$ 3,270	$ 3,262
Weighted Average Years to Maturity	3 years 1 month	3 years 7 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	$ 470	$ 494
Weighted Average Years to Maturity	2 years 7 months	3 years
Aaa/Aa/A | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 52	$ 42
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,800	$ 2,768
Weighted Average Years to Maturity	3 years 2 months	3 years 7 months
Baa
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 172	$ 113
Maximum Amount of Future Payments under Credit Default Swaps	$ 9,537	$ 8,877
Weighted Average Years to Maturity	4 years 10 months	4 years 8 months
Baa | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 9	$ 7
Maximum Amount of Future Payments under Credit Default Swaps	$ 806	$ 931
Weighted Average Years to Maturity	2 years 1 month	2 years 4 months
Baa | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 163	$ 106
Maximum Amount of Future Payments under Credit Default Swaps	$ 8,731	$ 7,946
Weighted Average Years to Maturity	5 years 1 month	5 years
Ba
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ (2)	$ (2)
Maximum Amount of Future Payments under Credit Default Swaps	$ 180	$ 155
Weighted Average Years to Maturity	3 years 8 months	4 years
Ba | 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	$ 180	$ 155
Weighted Average Years to Maturity	3 years 8 months	4 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	$ 25	$ 21
Maximum Amount of Future Payments under Credit Default Swaps	$ 390	$ 351
Weighted Average Years to Maturity	4 years 5 months	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	$ 60	$ 70
Weighted Average Years to Maturity	1 year 6 months	1 year 10 months
B | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 23	$ 20
Maximum Amount of Future Payments under Credit Default Swaps	$ 330	$ 281
Weighted Average Years to Maturity	5 years	5 years