Segment Data - Schedule of Segment Data (Parenthetical) (Details) - USD ($) $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2023		Sep. 30, 2022		Sep. 30, 2023		Sep. 30, 2022
Segment Reporting Information [Line Items]
Revenues from services	[2]	$ 4,675.6	[1]	$ 4,800.9	[1]	$ 14,284.0	[3]	$ 15,018.3	[3]
Franchise revenue		243.5		264.0		744.3		804.8
Franchise fees
Segment Reporting Information [Line Items]
Revenues from services		3.8		4.4		11.0		11.9
Americas | United States
Segment Reporting Information [Line Items]
Revenues from services		752.6		886.6		2,259.3		2,679.9
Americas | United States | Franchise fees
Segment Reporting Information [Line Items]
Revenues from services		3.1		3.7		8.9		9.7
Franchise revenue		$ 98.6		$ 121.9		$ 298.7		$ 360.8

[1] Certain service line revenues for the United States and Northern Europe were reclassified for the 2022 comparative period to align with the current period presentation. In the United States, $58.4 was reclassified from Outcome-Based Solutions and Consulting to Staffing and Interim. In Northern Europe, $24.8 was reclassified from Staffing and Interim to Outcome-Based Solutions and Consulting.

[2] Our consolidated revenues from services include fees received from our franchise offices of $3.8 and $4.4 for the three months ended September 30, 2023 and 2022, respectively, and $11.0 and $11.9 for the nine months ended September 30, 2023 and 2022, respectively. These fees are primarily based on revenues generated by the franchise offices, which were $243.5 and $264.0 for the three months ended September 30, 2023 and 2022, respectively, and $744.3 and $804.8 for the nine months ended September 30, 2023 and 2022, respectively.

[3] Certain service line revenues for the United States and Northern Europe were reclassified for the 2022 comparative period to align with the current period presentation. In the United States, $171.6 was reclassified from Outcome-Based Solutions and Consulting to Staffing and Interim. In Northern Europe, $90.0 was reclassified from Staffing and Interim to Outcome-Based Solutions and Consulting.