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	$ 23	$ 22
Maximum Amount of Future Payments under Credit Default Swaps	2,393	2,456
Weighted Average Years to Maturity	3 years 7 months	3 years 9 months 18 days
Aaa/Aa/A [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	9	13
Maximum Amount of Future Payments under Credit Default Swaps	787	817
Weighted Average Years to Maturity	1 year 9 months	2 years 3 months 18 days
Aaa/Aa/A [Member] | Single name credit default swaps (corporate) [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	2	3
Maximum Amount of Future Payments under Credit Default Swaps	137	167
Weighted Average Years to Maturity	2 years 9 months	3 years 2 months 12 days
Aaa/Aa/A [Member] | Credit default swaps referencing indices [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	7	10
Maximum Amount of Future Payments under Credit Default Swaps	650	650
Weighted Average Years to Maturity	1 year 7 months	2 years 1 month 6 days
Baa [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	13	9
Maximum Amount of Future Payments under Credit Default Swaps	1,560	1,603
Weighted Average Years to Maturity	4 years 6 months	4 years 6 months
Baa [Member] | Single name credit default swaps (corporate) [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	7	4
Maximum Amount of Future Payments under Credit Default Swaps	486	479
Weighted Average Years to Maturity	3 years 5 months	3 years 9 months 18 days
Baa [Member] | Credit default swaps referencing indices [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	6	5
Maximum Amount of Future Payments under Credit Default Swaps	1,074	1,124
Weighted Average Years to Maturity	5 years 0 months	4 years 9 months 18 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	10	0
Weighted Average Years to Maturity	5 years 1 month	0 years
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	10	0
Weighted Average Years to Maturity	5 years 1 month	0 years
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	1	0
Maximum Amount of Future Payments under Credit Default Swaps	36	36
Weighted Average Years to Maturity	5 years 1 month	5 years
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	1	0
Maximum Amount of Future Payments under Credit Default Swaps	$ 36	$ 36
Weighted Average Years to Maturity	5 years 1 month	5 years