Investments Loans by U.S. Region (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 2,909.8	[1]	$ 3,138.8	[1]
Loans, Percent of Geographic Region to Total Loans	100.00%	[1]	100.00%	[1]
Pacific
Investment Holdings [Line Items]
Total commercial mortgage loans	644.8	[1]	702.5	[1]
Loans, Percent of Geographic Region to Total Loans	22.10%	[1]	22.40%	[1]
South Atlantic
Investment Holdings [Line Items]
Total commercial mortgage loans	548.5	[1]	582.8	[1]
Loans, Percent of Geographic Region to Total Loans	18.90%	[1]	18.60%	[1]
Middle Atlantic
Investment Holdings [Line Items]
Total commercial mortgage loans	342.9	[1]	361.7	[1]
Loans, Percent of Geographic Region to Total Loans	11.80%	[1]	11.50%	[1]
East North Central
Investment Holdings [Line Items]
Total commercial mortgage loans	356.2	[1]	411.4	[1]
Loans, Percent of Geographic Region to Total Loans	12.20%	[1]	13.10%	[1]
West South Central
Investment Holdings [Line Items]
Total commercial mortgage loans	387.3	[1]	414.1	[1]
Loans, Percent of Geographic Region to Total Loans	13.30%	[1]	13.20%	[1]
Mountain
Investment Holdings [Line Items]
Total commercial mortgage loans	331.0	[1]	364.9	[1]
Loans, Percent of Geographic Region to Total Loans	11.40%	[1]	11.60%	[1]
New England
Investment Holdings [Line Items]
Total commercial mortgage loans	80.7	[1]	82.2	[1]
Loans, Percent of Geographic Region to Total Loans	2.80%	[1]	2.60%	[1]
West North Central
Investment Holdings [Line Items]
Total commercial mortgage loans	142.3	[1]	138.2	[1]
Loans, Percent of Geographic Region to Total Loans	4.90%	[1]	4.40%	[1]
East South Central
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 76.1	[1]	$ 81.0	[1]
Loans, Percent of Geographic Region to Total Loans	2.60%	[1]	2.60%	[1]

[1] Balances do not include allowance for mortgage loan credit losses.