Investments - Loans by Property Type (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 3,868.6	$ 3,514.1
Loans by property type percentage of total loans	100.00%	100.00%
Retail
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 1,351.8	$ 1,236.4
Loans by property type percentage of total loans	35.00%	35.20%
Industrial
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 866.6	$ 796.8
Loans by property type percentage of total loans	22.40%	22.70%
Apartments
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 623.9	$ 550.6
Loans by property type percentage of total loans	16.10%	15.70%
Office
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 581.6	$ 443.1
Loans by property type percentage of total loans	15.00%	12.60%
Hotel/Motel
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 178.8	$ 149.7
Loans by property type percentage of total loans	4.60%	4.20%
Mixed Use
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 47.4	$ 142.8
Loans by property type percentage of total loans	1.20%	4.10%
Other
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 218.5	$ 194.7
Loans by property type percentage of total loans	5.70%	5.50%