Investments (excluding Consolidated Investment Entities) - Mortgages by Year of Origination (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Total Commercial mortgage loans	$ 12,692.0	$ 11,728.3
2017
Investment [Line Items]
Total Commercial mortgage loans	1,538.7	0.0
2016
Investment [Line Items]
Total Commercial mortgage loans	2,365.7	2,349.6
2015
Investment [Line Items]
Total Commercial mortgage loans	2,038.8	2,066.1
2014
Investment [Line Items]
Total Commercial mortgage loans	1,846.0	1,860.3
2013
Investment [Line Items]
Total Commercial mortgage loans	1,919.9	1,953.1
2012
Investment [Line Items]
Total Commercial mortgage loans	1,070.6	1,241.4
2011 and prior
Investment [Line Items]
Total Commercial mortgage loans	$ 1,912.3	$ 2,257.8