Investments (excluding Consolidated Investment Entities) - Loans by Property Type (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Total Commercial mortgage loans	$ 10,369.5	$ 9,796.9
Loans by property type percentage of total loans	100.00%	100.00%
Retail
Investment Holdings [Line Items]
Total Commercial mortgage loans	$ 3,717.1	$ 3,408.4
Loans by property type percentage of total loans	35.80%	34.80%
Industrial
Investment Holdings [Line Items]
Total Commercial mortgage loans	$ 2,363.8	$ 2,283.0
Loans by property type percentage of total loans	22.80%	23.30%
Apartments
Investment Holdings [Line Items]
Total Commercial mortgage loans	$ 1,823.0	$ 1,680.7
Loans by property type percentage of total loans	17.60%	17.20%
Office
Investment Holdings [Line Items]
Total Commercial mortgage loans	$ 1,406.5	$ 1,246.5
Loans by property type percentage of total loans	13.60%	12.70%
Hotel/Motel
Investment Holdings [Line Items]
Total Commercial mortgage loans	$ 399.3	$ 382.7
Loans by property type percentage of total loans	3.90%	3.90%
Mixed Use
Investment Holdings [Line Items]
Total Commercial mortgage loans	$ 106.2	$ 346.5
Loans by property type percentage of total loans	1.00%	3.50%
Other
Investment Holdings [Line Items]
Total Commercial mortgage loans	$ 553.6	$ 449.1
Loans by property type percentage of total loans	5.30%	4.60%