Investments and Derivative Instruments (Details 9) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	$ 6,875		$ 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	148		94
Percent of Total	2.15%		1.64%
Middle Atlantic [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	499		508
Percent of Total	7.26%		8.87%
Mountain [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	124		125
Percent of Total	1.80%		2.18%
New England [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	336		294
Percent of Total	4.89%		5.13%
Pacific [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	1,976		1,690
Percent of Total	28.74%		29.50%
South Atlantic [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	1,434		1,149
Percent of Total	20.86%		20.06%
West North Central [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	16		30
Percent of Total	0.23%		0.52%
West South Central [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	414		224
Percent of Total	6.02%		3.91%
Other [Member]
Schedule of Available-for-sale Securities Mortgage Loans [Line Items]
Carrying Value	$ 1,928	[1]	$ 1,614	[1]
Percent of Total	28.04%	[1]	28.18%	[1]

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