Investments - Loans by Property Type (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 3,732.1	$ 3,514.1
Loans by property type percentage of total loans	100.00%	100.00%
Retail
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 1,341.4	$ 1,236.4
Loans by property type percentage of total loans	35.90%	35.20%
Industrial
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 824.3	$ 796.8
Loans by property type percentage of total loans	22.10%	22.70%
Apartments
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 609.8	$ 550.6
Loans by property type percentage of total loans	16.30%	15.70%
Office
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 512.8	$ 443.1
Loans by property type percentage of total loans	13.70%	12.60%
Hotel/Motel
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 158.9	$ 149.7
Loans by property type percentage of total loans	4.30%	4.20%
Mixed Use
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 47.7	$ 142.8
Loans by property type percentage of total loans	1.30%	4.10%
Other
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 237.2	$ 194.7
Loans by property type percentage of total loans	6.40%	5.50%