Investments - Mortgages by Year of Origination (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Total commercial mortgage loans	$ 4,746.3	$ 4,255.6
Year of Origination 2017
Investment [Line Items]
Total commercial mortgage loans	614.0	0.0
Year of Origination 2016
Investment [Line Items]
Total commercial mortgage loans	886.0	874.3
Year of Origination 2015
Investment [Line Items]
Total commercial mortgage loans	727.2	729.1
Year of Origination 2014
Investment [Line Items]
Total commercial mortgage loans	548.2	548.0
Year of Origination 2013
Investment [Line Items]
Total commercial mortgage loans	679.2	685.1
Year of Origination 2012
Investment [Line Items]
Total commercial mortgage loans	593.0	681.1
Year of Origination 2011 and prior
Investment [Line Items]
Total commercial mortgage loans	$ 698.7	$ 738.0