Investments - Mortgages by Year of Origination (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Total commercial mortgage loans	$ 4,849.1	$ 4,255.6
Year of Origination 2017
Investment [Line Items]
Total commercial mortgage loans	815.3	0.0
Year of Origination 2016
Investment [Line Items]
Total commercial mortgage loans	885.1	874.3
Year of Origination 2015
Investment [Line Items]
Total commercial mortgage loans	718.0	729.1
Year of Origination 2014
Investment [Line Items]
Total commercial mortgage loans	543.1	548.0
Year of Origination 2013
Investment [Line Items]
Total commercial mortgage loans	673.0	685.1
Year of Origination 2012
Investment [Line Items]
Total commercial mortgage loans	576.9	681.1
Year of Origination 2011 and prior
Investment [Line Items]
Total commercial mortgage loans	$ 637.7	$ 738.0