Investments - Mortgages by Year of Origination (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Investment [Line Items]
Total commercial mortgage loans	$ 4,110.4	$ 3,730.3
Year of Origination 2016
Investment [Line Items]
Total commercial mortgage loans	662.0	0.0
Year of Origination 2015
Investment [Line Items]
Total commercial mortgage loans	731.0	745.3
Year of Origination 2014
Investment [Line Items]
Total commercial mortgage loans	549.1	558.0
Year of Origination 2013
Investment [Line Items]
Total commercial mortgage loans	691.2	709.2
Year of Origination 2012
Investment [Line Items]
Total commercial mortgage loans	689.6	748.2
Year of Origination 2011
Investment [Line Items]
Total commercial mortgage loans	450.9	553.2
Year of Origination 2010 and prior
Investment [Line Items]
Total commercial mortgage loans	$ 336.6	$ 416.4