Goodwill and Other Intangible Assets - Goodwill by Reporting Unit (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill	$ 1,360.0	$ 1,343.0
United States
Goodwill [Line Items]
Goodwill	532.0	532.0
Germany
Goodwill [Line Items]
Goodwill	139.1	135.4
Netherlands
Goodwill [Line Items]
Goodwill	129.9	126.5
United Kingdom
Goodwill [Line Items]
Goodwill	92.5	89.2
France
Goodwill [Line Items]
Goodwill	74.1	76.3
Right Management
Goodwill [Line Items]
Goodwill	62.1	62.1
Other reporting units
Goodwill [Line Items]
Goodwill	$ 330.3	$ 321.5