Investments and Derivative Instruments (Details 9) (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
Carrying Value	$ 6,863		$ 5,728
Percent of Total	100.00%		100.00%
East North Central [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	145		94
Percent of Total	2.10%		1.60%
Middle Atlantic [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	498		508
Percent of Total	7.30%		8.90%
Mountain [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	100		125
Percent of Total	1.50%		2.20%
New England [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	335		294
Percent of Total	4.90%		5.10%
Pacific [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	2,054		1,690
Percent of Total	29.90%		29.50%
South Atlantic [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	1,388		1,149
Percent of Total	20.20%		20.10%
West North Central [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	16		30
Percent of Total	0.20%		0.50%
West South Central [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	435		224
Percent of Total	6.30%		3.90%
Other [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	$ 1,892	[1]	$ 1,614	[1]
Percent of Total	27.60%	[1]	28.20%	[1]

[1] Primarily represents loans collateralized by multiple properties in various regions.