Basis of Presentation and Accounting Policies - Summary of Reporting Units' Estimated Fair Values and Carrying Values, Determined as Part of Annual Goodwill Impairment Test (Details) $ in Millions	Sep. 30, 2020 USD ($)
Southern Europe | France
Goodwill [Line Items]
Estimated fair values	$ 2,367.8
Carrying values	1,320.7
Americas | United States
Goodwill [Line Items]
Estimated fair values	1,194.0
Carrying values	758.2
Americas | Other Americas | Canada
Goodwill [Line Items]
Estimated fair values	153.2
Carrying values	84.9
Right Management
Goodwill [Line Items]
Estimated fair values	383.3
Carrying values	117.9
Northern Europe | United Kingdom
Goodwill [Line Items]
Estimated fair values	360.9
Carrying values	321.5
Northern Europe | Netherlands
Goodwill [Line Items]
Estimated fair values	84.5
Carrying values	$ 81.8