Investments and Derivative Instruments (Details 10) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Document Period End Date	Sep. 30, 2012
Mortgage Loans by Property Type
Carrying Value	$ 6,863	$ 5,728
Percent of Total	100.00%	100.00%
Agricultural [Member]
Mortgage Loans by Property Type
Carrying Value	155	249
Percent of Total	2.30%	4.30%
Industrial [Member]
Mortgage Loans by Property Type
Carrying Value	2,082	1,747
Percent of Total	30.30%	30.50%
Lodging [Member]
Mortgage Loans by Property Type
Carrying Value	82	93
Percent of Total	1.20%	1.60%
Multifamily [Member]
Mortgage Loans by Property Type
Carrying Value	1,329	1,070
Percent of Total	19.40%	18.70%
Office [Member]
Mortgage Loans by Property Type
Carrying Value	1,517	1,078
Percent of Total	22.10%	18.80%
Retail [Member]
Mortgage Loans by Property Type
Carrying Value	1,449	1,234
Percent of Total	21.10%	21.50%
Other [Member]
Mortgage Loans by Property Type
Carrying Value	$ 249	$ 257
Percent of Total	3.60%	4.60%