Goodwill and Other Intangible Assets - Schedule of Goodwill Balances by Reporting Unit (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Goodwill [Line Items]
Goodwill	$ 1,198.6	$ 1,260.1
Jefferson Wells
Goodwill [Line Items]
Goodwill	55.5
Netherlands
Goodwill [Line Items]
Goodwill	114.5
Americas | United States
Goodwill [Line Items]
Goodwill	490.2	490.3
Americas | United States | Jefferson Wells
Goodwill [Line Items]
Goodwill	545.7	545.8
Northern Europe | Netherlands
Goodwill [Line Items]
Goodwill	114.5	109.5
Northern Europe | United Kingdom
Goodwill [Line Items]
Goodwill	94.7	97.3
Northern Europe | Germany
Goodwill [Line Items]
Goodwill		67.2
Southern Europe | France
Goodwill [Line Items]
Goodwill	70.3	67.3
Other Reporting Units
Goodwill [Line Items]
Goodwill	311.3	310.9
Reportable segments | Americas
Goodwill [Line Items]
Goodwill	534.3	535.6
Reportable segments | Northern Europe
Goodwill [Line Items]
Goodwill	309.1	374.6
Reportable segments | Southern Europe
Goodwill [Line Items]
Goodwill	149.9	144.8
Reportable segments | Right Management
Goodwill [Line Items]
Goodwill	$ 62.1	$ 62.1