Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 180	$ 65
Maximum Amount of Future Payments under Credit Default Swaps	$ 12,645	$ 10,311
Weighted Average Years to Maturity	4 years 5 months	4 years 1 month
Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 48	$ 12
Maximum Amount of Future Payments under Credit Default Swaps	$ 3,262	$ 2,296
Weighted Average Years to Maturity	3 years 7 months	3 years 2 months
Aaa/Aa/A | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 6	$ 6
Maximum Amount of Future Payments under Credit Default Swaps	$ 494	$ 661
Weighted Average Years to Maturity	3 years	2 years 6 months
Aaa/Aa/A | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 42	$ 6
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,768	$ 1,635
Weighted Average Years to Maturity	3 years 7 months	3 years 5 months
Baa
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 113	$ 45
Maximum Amount of Future Payments under Credit Default Swaps	$ 8,877	$ 7,212
Weighted Average Years to Maturity	4 years 8 months	4 years 5 months
Baa | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 7	$ 8
Maximum Amount of Future Payments under Credit Default Swaps	$ 931	$ 1,349
Weighted Average Years to Maturity	2 years 4 months	2 years 6 months
Baa | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 106	$ 37
Maximum Amount of Future Payments under Credit Default Swaps	$ 7,946	$ 5,863
Weighted Average Years to Maturity	5 years	4 years 10 months
Ba
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ (2)	$ (3)
Maximum Amount of Future Payments under Credit Default Swaps	$ 155	$ 164
Weighted Average Years to Maturity	4 years	1 year 6 months
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	$ 155	$ 64
Weighted Average Years to Maturity	4 years	2 years 4 months
Ba | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 0	$ (1)
Maximum Amount of Future Payments under Credit Default Swaps	$ 0	$ 100
Weighted Average Years to Maturity	0 years	1 year
B [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 21	$ 11
Maximum Amount of Future Payments under Credit Default Swaps	$ 351	$ 639
Weighted Average Years to Maturity	4 years 4 months	4 years 11 months
B [Member] | Single name credit default swaps (3)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 1	$ 0
Maximum Amount of Future Payments under Credit Default Swaps	$ 70	$ 0
Weighted Average Years to Maturity	1 year 10 months	0 years
B [Member] | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 20	$ 11
Maximum Amount of Future Payments under Credit Default Swaps	$ 281	$ 639
Weighted Average Years to Maturity	5 years	4 years 11 months