Derivatives (Credit Derivatives) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 82	$ 74
Maximum Amount of Future Payments under Credit Default Swaps	9,644	8,879
Weighted Average Years to Maturity	3 years 6 months 0 days	3 years 7 months 6 days
Aaa/Aa/A [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	35	52
Maximum Amount of Future Payments under Credit Default Swaps	3,646	3,490
Weighted Average Years to Maturity	1 year 9 months 18 days	2 years 2 months 12 days
Aaa/Aa/A [Member] | Single name credit default swaps (corporate) [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	9	10
Maximum Amount of Future Payments under Credit Default Swaps	732	777
Weighted Average Years to Maturity	2 years 4 months 24 days	2 years 8 months 12 days
Aaa/Aa/A [Member] | Credit default swaps referencing indices [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	26	42
Maximum Amount of Future Payments under Credit Default Swaps	2,914	2,713
Weighted Average Years to Maturity	1 year 8 months 12 days	2 years 1 month 6 days
Baa [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	36	19
Maximum Amount of Future Payments under Credit Default Swaps	5,568	5,064
Weighted Average Years to Maturity	4 years 7 months 6 days	4 years 6 months 0 days
Baa [Member] | Single name credit default swaps (corporate) [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	12	8
Maximum Amount of Future Payments under Credit Default Swaps	1,668	1,314
Weighted Average Years to Maturity	3 years 8 months 12 days	3 years 4 months 24 days
Baa [Member] | Credit default swaps referencing indices [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	24	11
Maximum Amount of Future Payments under Credit Default Swaps	3,900	3,750
Weighted Average Years to Maturity	4 years 10 months 24 days	4 years 10 months 24 days
Ba [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	0	0
Maximum Amount of Future Payments under Credit Default Swaps	55	25
Weighted Average Years to Maturity	3 years 9 months 18 days	2 years 8 months 12 days
Ba [Member] | Single name credit default swaps (corporate) [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	0	0
Maximum Amount of Future Payments under Credit Default Swaps	55	25
Weighted Average Years to Maturity	3 years 9 months 18 days	2 years 8 months 12 days
Ba [Member] | Credit default swaps referencing indices [Member]
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 [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	11	3
Maximum Amount of Future Payments under Credit Default Swaps	375	300
Weighted Average Years to Maturity	4 years 10 months 24 days	4 years 10 months 24 days
B [Member] | Single name credit default swaps (corporate) [Member]
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 [Member] | Credit default swaps referencing indices [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	11	3
Maximum Amount of Future Payments under Credit Default Swaps	$ 375	$ 300
Weighted Average Years to Maturity	4 years 10 months 24 days	4 years 10 months 24 days