Revenue Recognition - Disaggregation of Revenue (Parenthetical) (Details) - USD ($) $ in Millions		3 Months Ended				9 Months Ended				12 Months Ended
		Sep. 30, 2023	[1]	Sep. 30, 2022	[1]	Sep. 30, 2023		Sep. 30, 2022	[3]	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
Revenue	[2]	$ 4,675.6		$ 4,800.9		$ 14,284.0	[3]	$ 15,018.3
Outcome-Based Solutions and Consulting | United States
Disaggregation of Revenue [Line Items]
Revenue						171.6				$ 58.4
Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue						90.0
Northern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue		78.4		76.1		250.7	[3]	249.0
Northern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue		$ 773.8		$ 815.3		$ 2,389.2	[3]	$ 2,627.4		$ 24.8

[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.