Investments (Commercial Mortgage Loan Portfolio By Property Type, By Geographic Region Within The U.S. And By U.S. State) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Commercial mortgage loans, net	$ 5,149.0	$ 4,902.3
Commercial mortgage loans, Percentage	100.00%	100.00%
California [Member]
Commercial mortgage loans, net	1,418.4	1,332.0
Commercial mortgage loans, Percentage	27.50%	27.20%
Texas [Member]
Commercial mortgage loans, net	574.0	550.8
Commercial mortgage loans, Percentage	11.10%	11.20%
Georgia [Member]
Commercial mortgage loans, net	304.3	270.1
Commercial mortgage loans, Percentage	5.90%	5.50%
Florida [Member]
Commercial mortgage loans, net	316.0	305.3
Commercial mortgage loans, Percentage	6.20%	6.20%
Other States [Member]
Commercial mortgage loans, net	2,536.3	2,444.1
Commercial mortgage loans, Percentage	49.30%	49.90%
Pacific [Member]
Commercial mortgage loans, net	1,811.2	1,699.3
Commercial mortgage loans, Percentage	35.20%	34.70%
South Atlantic [Member]
Commercial mortgage loans, net	1,028.9	953.8
Commercial mortgage loans, Percentage	20.00%	19.50%
Mountain [Member]
Commercial mortgage loans, net	609.1	585.8
Commercial mortgage loans, Percentage	11.80%	11.90%
West South Central [Member]
Commercial mortgage loans, net	629.6	605.3
Commercial mortgage loans, Percentage	12.20%	12.30%
East North Central [Member]
Commercial mortgage loans, net	420.3	393.4
Commercial mortgage loans, Percentage	8.20%	8.00%
Middle Atlantic [Member]
Commercial mortgage loans, net	241.8	243.8
Commercial mortgage loans, Percentage	4.70%	5.00%
West North Central [Member]
Commercial mortgage loans, net	183.9	184.8
Commercial mortgage loans, Percentage	3.60%	3.80%
East South Central [Member]
Commercial mortgage loans, net	131.7	129.9
Commercial mortgage loans, Percentage	2.50%	2.60%
New England [Member]
Commercial mortgage loans, net	92.5	106.2
Commercial mortgage loans, Percentage	1.80%	2.20%
Retail [Member]
Commercial mortgage loans, net	2,492.2	2,457.8
Commercial mortgage loans, Percentage	48.40%	50.10%
Office [Member]
Commercial mortgage loans, net	944.7	911.1
Commercial mortgage loans, Percentage	18.30%	18.60%
Industrial [Member]
Commercial mortgage loans, net	967.4	900.4
Commercial mortgage loans, Percentage	18.80%	18.40%
Hotels and Motels [Member]
Commercial mortgage loans, net	297.5	241.9
Commercial mortgage loans, Percentage	5.80%	4.90%
Commercial [Member]
Commercial mortgage loans, net	199.4	187.1
Commercial mortgage loans, Percentage	3.90%	3.80%
Apartment and Other [Member]
Commercial mortgage loans, net	$ 247.8	$ 204.0
Commercial mortgage loans, Percentage	4.80%	4.20%