Investments and Derivative Instruments (Details 11) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Variable Interest Entity [Line Items]
Document Period End Date	Mar. 31, 2013
Variable Interest Entities Primary Beneficiary
Total Assets	$ 226	$ 258
Total Liabilities	56	89
Maximum Exposure to Loss	174	174
Collateralized Debt Obligations [Member] | Consolidated VIEs [Member]
Variable Interest Entities Primary Beneficiary
Total Assets	59	89
Total Liabilities	55	88
Maximum Exposure to Loss	7	7
Investment Fund [Member] | Consolidated VIEs [Member]
Variable Interest Entities Primary Beneficiary
Total Assets	163	163
Total Liabilities	0	0
Maximum Exposure to Loss	164	162
Limited partnerships [Member] | Consolidated VIEs [Member]
Variable Interest Entities Primary Beneficiary
Total Liabilities	1	1
Maximum Exposure to Loss	3	5
Variable Interest Entity Assets Included in Limited Partnerships and Other Alternative Investments	$ 4	$ 6