Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 94	$ 65
Maximum Amount of Future Payments under Credit Default Swaps	$ 10,729	$ 10,311
Weighted Average Years to Maturity	4 years 4 months	4 years 1 month
Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 22	$ 12
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,767	$ 2,296
Weighted Average Years to Maturity	3 years 5 months	3 years 2 months
Aaa/Aa/A | Single name credit default swaps (corporate)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 6	$ 6
Maximum Amount of Future Payments under Credit Default Swaps	$ 630	$ 661
Weighted Average Years to Maturity	2 years 10 months	2 years 6 months
Aaa/Aa/A | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 16	$ 6
Maximum Amount of Future Payments under Credit Default Swaps	$ 2,137	$ 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	$ 60	$ 45
Maximum Amount of Future Payments under Credit Default Swaps	$ 7,436	$ 7,212
Weighted Average Years to Maturity	4 years 8 months	4 years 5 months
Baa | Single name credit default swaps (corporate)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 5	$ 8
Maximum Amount of Future Payments under Credit Default Swaps	$ 1,158	$ 1,349
Weighted Average Years to Maturity	2 years 5 months	2 years 6 months
Baa | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 55	$ 37
Maximum Amount of Future Payments under Credit Default Swaps	$ 6,278	$ 5,863
Weighted Average Years to Maturity	5 years 1 month	4 years 10 months
Ba
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 0	$ (3)
Maximum Amount of Future Payments under Credit Default Swaps	$ 165	$ 164
Weighted Average Years to Maturity	1 year 4 months	1 year 6 months
Ba | Single name credit default swaps (corporate)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 0	$ (2)
Maximum Amount of Future Payments under Credit Default Swaps	$ 65	$ 64
Weighted Average Years to Maturity	2 years 1 month	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	$ 100	$ 100
Weighted Average Years to Maturity	8 months	1 year
B
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 12	$ 11
Maximum Amount of Future Payments under Credit Default Swaps	$ 361	$ 639
Weighted Average Years to Maturity	4 years 7 months	4 years 11 months
B | Single name credit default swaps (corporate)
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 default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 12	$ 11
Maximum Amount of Future Payments under Credit Default Swaps	$ 361	$ 639
Weighted Average Years to Maturity	4 years 7 months	4 years 11 months