Investments Loans by Property Type (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 Property Type to Total Loans	100.00%		100.00%
Industrial
Investment Holdings [Line Items]
Total commercial mortgage loans	1,144.7		1,223.2
Loans, Percent of Property Type to Total Loans	39.40%		39.00%
Retail
Investment Holdings [Line Items]
Total commercial mortgage loans	740.4		807.4
Loans, Percent of Property Type to Total Loans	25.40%		25.70%
Office
Investment Holdings [Line Items]
Total commercial mortgage loans	475.8		542.2
Loans, Percent of Property Type to Total Loans	16.40%		17.30%
Apartments
Investment Holdings [Line Items]
Total commercial mortgage loans	350.1		371.5
Loans, Percent of Property Type to Total Loans	12.00%		11.80%
Hotel/Motel
Investment Holdings [Line Items]
Total commercial mortgage loans	128.6		129.6
Loans, Percent of Property Type to Total Loans	4.40%		4.10%
Mixed use
Investment Holdings [Line Items]
Total commercial mortgage loans	16.7		12.6
Loans, Percent of Property Type to Total Loans	0.60%		0.40%
Other
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 53.5		$ 52.3
Loans, Percent of Property Type to Total Loans	1.80%		1.70%

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