Investments - Mortgages by Year of Origination (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Total commercial mortgage loans	$ 4,868.5	$ 4,255.6
Year of Origination 2017
Investment [Line Items]
Total commercial mortgage loans	917.4	0.0
Year of Origination 2016
Investment [Line Items]
Total commercial mortgage loans	877.9	874.3
Year of Origination 2015
Investment [Line Items]
Total commercial mortgage loans	710.9	729.1
Year of Origination 2014
Investment [Line Items]
Total commercial mortgage loans	542.2	548.0
Year of Origination 2013
Investment [Line Items]
Total commercial mortgage loans	661.0	685.1
Year of Origination 2012
Investment [Line Items]
Total commercial mortgage loans	536.0	681.1
Year of Origination 2011 and prior
Investment [Line Items]
Total commercial mortgage loans	$ 623.1	$ 738.0