Investments - Mortgages by Year of Origination (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Total commercial mortgage loans	$ 3,931.2	$ 3,730.3
Year of Origination 2016
Investment [Line Items]
Total commercial mortgage loans	275.5	0.0
Year of Origination 2015
Investment [Line Items]
Total commercial mortgage loans	742.7	745.3
Year of Origination 2014
Investment [Line Items]
Total commercial mortgage loans	557.3	558.0
Year of Origination 2013
Investment [Line Items]
Total commercial mortgage loans	703.7	709.2
Year of Origination 2012
Investment [Line Items]
Total commercial mortgage loans	742.8	748.2
Year of Origination 2011
Investment [Line Items]
Total commercial mortgage loans	522.0	553.2
Year of Origination 2010 and prior
Investment [Line Items]
Total commercial mortgage loans	$ 387.2	$ 416.4