Derivatives (Credit Derivatives) (Details) (USD $)	3 Months Ended	12 Months Ended	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 186,000,000	$ 175,000,000
Maximum Amount of Future Payments under Credit Default Swaps	10,206,000,000	10,527,000,000
Weighted Average Years to Maturity	4 years	3 years 11 months
Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	15,000,000	20,000,000
Maximum Amount of Future Payments under Credit Default Swaps	2,136,000,000	2,377,000,000
Weighted Average Years to Maturity	2 years 7 months	2 years 7 months
Aaa/Aa/A | Single name credit default swaps (corporate)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	8,000,000	10,000,000
Maximum Amount of Future Payments under Credit Default Swaps	636,000,000	677,000,000
Weighted Average Years to Maturity	2 years 4 months	2 years 5 months
Aaa/Aa/A | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	7,000,000	10,000,000
Maximum Amount of Future Payments under Credit Default Swaps	1,500,000,000	1,700,000,000
Weighted Average Years to Maturity	2 years 8 months	2 years 7 months
Baa
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	120,000,000	117,000,000
Maximum Amount of Future Payments under Credit Default Swaps	7,297,000,000	7,365,000,000
Weighted Average Years to Maturity	4 years 5 months	4 years 3 months
Baa | Single name credit default swaps (corporate)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	23,000,000	23,000,000
Maximum Amount of Future Payments under Credit Default Swaps	1,555,000,000	1,591,000,000
Weighted Average Years to Maturity	2 years 7 months	2 years 9 months
Baa | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	97,000,000	94,000,000
Maximum Amount of Future Payments under Credit Default Swaps	5,742,000,000	5,774,000,000
Weighted Average Years to Maturity	4 years 11 months	4 years 8 months
Ba
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	1,000,000	(1,000,000)
Maximum Amount of Future Payments under Credit Default Swaps	160,000,000	160,000,000
Weighted Average Years to Maturity	2 years 1 month	2 years 5 months
Ba | Single name credit default swaps (corporate)
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	1,000,000	0
Maximum Amount of Future Payments under Credit Default Swaps	60,000,000	60,000,000
Weighted Average Years to Maturity	2 years 8 months	3 years
Ba | Credit default swaps referencing indices
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	0	(1,000,000)
Maximum Amount of Future Payments under Credit Default Swaps	100,000,000	100,000,000
Weighted Average Years to Maturity	1 year 10 months		2 years
B
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	50,000,000	39,000,000
Maximum Amount of Future Payments under Credit Default Swaps	613,000,000	625,000,000
Weighted Average Years to Maturity	4 years 8 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	50,000,000	39,000,000
Maximum Amount of Future Payments under Credit Default Swaps	$ 613,000,000	$ 625,000,000
Weighted Average Years to Maturity	4 years 8 months	4 years 11 months