Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 199	$ 180
Maximum Amount of Future Payments under Credit Default Swaps	$ 12,369	$ 12,645
Weighted Average Years to Maturity	4 years 7 months	4 years 5 months
Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 51	$ 48
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,984	$ 3,262
Weighted Average Years to Maturity	3 years 4 months	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	$ 484	$ 494
Weighted Average Years to Maturity	2 years 10 months	3 years
Aaa/Aa/A | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 43	$ 42
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,500	$ 2,768
Weighted Average Years to Maturity	3 years 5 months	3 years 7 months
Baa
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 128	$ 113
Maximum Amount of Future Payments under Credit Default Swaps	$ 8,865	$ 8,877
Weighted Average Years to Maturity	5 years	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	$ 881	$ 931
Weighted Average Years to Maturity	2 years 2 months	2 years 4 months
Baa | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 119	$ 106
Maximum Amount of Future Payments under Credit Default Swaps	$ 7,984	$ 7,946
Weighted Average Years to Maturity	5 years 4 months	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 11 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 11 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	$ 22	$ 21
Maximum Amount of Future Payments under Credit Default Swaps	$ 340	$ 351
Weighted Average Years to Maturity	4 years 6 months	4 years 4 months
B | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 1	$ 1
Maximum Amount of Future Payments under Credit Default Swaps	$ 70	$ 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	$ 21	$ 20
Maximum Amount of Future Payments under Credit Default Swaps	$ 270	$ 281
Weighted Average Years to Maturity	5 years 2 months	5 years