Goodwill and Other Intangible Assets - Goodwill by Reporting Unit (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Goodwill	$ 1,293.6	$ 1,297.1
United States
Goodwill [Line Items]
Goodwill	532.0	532.0
Germany
Goodwill [Line Items]
Goodwill	126.9	129.2
Netherlands
Goodwill [Line Items]
Goodwill	109.6	112.0
United Kingdom
Goodwill [Line Items]
Goodwill	95.7	93.7
France
Goodwill [Line Items]
Goodwill	67.4	68.9
Right Management
Goodwill [Line Items]
Goodwill	62.1	62.1
Other reporting units
Goodwill [Line Items]
Goodwill	$ 299.9	$ 299.2