Investments - Mortgages by Year of Origination (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Investment [Line Items]
Total commercial mortgage loans	$ 4,030.1	$ 3,730.3
Year of Origination 2016
Investment [Line Items]
Total commercial mortgage loans	513.6	0.0
Year of Origination 2015
Investment [Line Items]
Total commercial mortgage loans	739.8	745.3
Year of Origination 2014
Investment [Line Items]
Total commercial mortgage loans	556.1	558.0
Year of Origination 2013
Investment [Line Items]
Total commercial mortgage loans	697.1	709.2
Year of Origination 2012
Investment [Line Items]
Total commercial mortgage loans	704.6	748.2
Year of Origination 2011
Investment [Line Items]
Total commercial mortgage loans	466.8	553.2
Year of Origination 2010 and prior
Investment [Line Items]
Total commercial mortgage loans	$ 352.1	$ 416.4