Investments - Loans by Property Type (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 2,874.0	[1]	$ 2,374.8	[1]
Loans by property type percentage of total loans	100.00%	[1]	100.00%	[1]
Industrial
Investment Holdings [Line Items]
Total commercial mortgage loans	1,035.2	[1]	956.4	[1]
Loans by property type percentage of total loans	36.00%	[1]	40.30%	[1]
Retail
Investment Holdings [Line Items]
Total commercial mortgage loans	824.0	[1]	544.7	[1]
Loans by property type percentage of total loans	28.70%	[1]	22.90%	[1]
Office
Investment Holdings [Line Items]
Total commercial mortgage loans	427.0	[1]	351.5	[1]
Loans by property type percentage of total loans	14.80%	[1]	14.80%	[1]
Apartments
Investment Holdings [Line Items]
Total commercial mortgage loans	298.7	[1]	281.7	[1]
Loans by property type percentage of total loans	10.40%	[1]	11.90%	[1]
Hotel/Motel
Investment Holdings [Line Items]
Total commercial mortgage loans	92.1	[1]	132.7	[1]
Loans by property type percentage of total loans	3.20%	[1]	5.60%	[1]
Mixed Use
Investment Holdings [Line Items]
Total commercial mortgage loans	34.2	[1]	0.9	[1]
Loans by property type percentage of total loans	1.20%	[1]	0.00%	[1]
Other
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 162.8	[1]	$ 106.9	[1]
Loans by property type percentage of total loans	5.70%	[1]	4.50%	[1]

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