Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of Revenue [Line Items]
Revenue	[1]	$ 18,914.5	$ 19,827.5	$ 20,724.4
Intercompany Eliminations
Disaggregation of Revenue [Line Items]
Revenue		(85.9)	(80.1)	(37.7)
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue			19,378.0
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		445.7	529.6
Reportable subsegments
Disaggregation of Revenue [Line Items]
Revenue		19,000.4	19,907.6
Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		4,308.6	4,787.2
Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		106.2	148.5
Americas | United States
Disaggregation of Revenue [Line Items]
Revenue		2,961.6	3,499.3
Americas | United States | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		2,885.0	3,380.9
Americas | United States | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		76.6	118.4
Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue		1,453.2	1,436.4
Americas | Other Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		1,423.6	1,406.3
Americas | Other Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		29.6	30.1
Americas | Reportable subsegments
Disaggregation of Revenue [Line Items]
Revenue		4,414.8	4,935.7	4,263.7
Americas | Reportable subsegments | United States
Disaggregation of Revenue [Line Items]
Revenue	[1]	2,961.6	3,499.3	2,743.3
Americas | Reportable subsegments | Other Americas
Disaggregation of Revenue [Line Items]
Revenue		1,453.2	1,436.4	1,520.4
Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		8,363.4	8,385.5
Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		151.9	150.8
Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue		4,867.1	4,785.0
Southern Europe | France | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		4,814.5	4,732.3
Southern Europe | France | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		52.6	52.7
Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue		1,708.8	1,706.9
Southern Europe | Italy | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		1,657.8	1,659.4
Southern Europe | Italy | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		51.0	47.5
Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue		1,939.4	2,044.4
Southern Europe | Other Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		1,891.1	1,993.8
Southern Europe | Other Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		48.3	50.6
Southern Europe | Reportable subsegments
Disaggregation of Revenue [Line Items]
Revenue		8,515.3	8,536.3	9,346.8
Southern Europe | Reportable subsegments | France
Disaggregation of Revenue [Line Items]
Revenue		4,867.1	4,785.0	5,171.3
Southern Europe | Reportable subsegments | Italy
Disaggregation of Revenue [Line Items]
Revenue		1,708.8	1,706.9	1,795.4
Southern Europe | Reportable subsegments | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue		1,939.4	2,044.4	2,380.1
Northern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		3,627.5	3,902.9
Northern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		120.5	145.4
Northern Europe | Reportable subsegments
Disaggregation of Revenue [Line Items]
Revenue		3,748.0	4,048.3	4,670.5
APME | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		2,255.2	2,302.4
APME | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		67.1	84.9
APME | Reportable subsegments
Disaggregation of Revenue [Line Items]
Revenue		2,322.3	2,387.3	2,481.1
Northern Europe And APME | Reportable subsegments
Disaggregation of Revenue [Line Items]
Revenue		19,000.4	19,907.6	$ 20,762.1
Reportable Segments
Disaggregation of Revenue [Line Items]
Revenue		18,554.7
Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue		120.5
Staffing and Interim | Americas
Disaggregation of Revenue [Line Items]
Revenue		3,995.1	4,444.4
Staffing and Interim | Americas | United States
Disaggregation of Revenue [Line Items]
Revenue		2,647.7	3,115.1
Staffing and Interim | Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue		1,347.4	1,329.3
Staffing and Interim | Southern Europe
Disaggregation of Revenue [Line Items]
Revenue		7,592.0	7,657.5
Staffing and Interim | Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue		4,454.7	4,421.8
Staffing and Interim | Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue		1,590.1	1,600.7
Staffing and Interim | Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue		1,547.2	1,635.0
Staffing and Interim | Northern Europe
Disaggregation of Revenue [Line Items]
Revenue		3,160.2	3,446.9
Staffing and Interim | APME
Disaggregation of Revenue [Line Items]
Revenue		1,781.8	1,809.6
Staffing and Interim | Northern Europe And APME
Disaggregation of Revenue [Line Items]
Revenue		16,529.1	17,358.4
Outcome-Based Solutions and Consulting | United States
Disaggregation of Revenue [Line Items]
Revenue		252.6
Outcome-Based Solutions and Consulting | Americas
Disaggregation of Revenue [Line Items]
Revenue		67.7	73.6
Outcome-Based Solutions and Consulting | Americas | United States
Disaggregation of Revenue [Line Items]
Revenue		18.3	21.7
Outcome-Based Solutions and Consulting | Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue		49.4	51.9
Outcome-Based Solutions and Consulting | Southern Europe
Disaggregation of Revenue [Line Items]
Revenue		625.5	636.6
Outcome-Based Solutions and Consulting | Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue		283.5	268.0
Outcome-Based Solutions and Consulting | Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue		36.9	31.9
Outcome-Based Solutions and Consulting | Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue		305.1	336.7
Outcome-Based Solutions and Consulting | Northern Europe
Disaggregation of Revenue [Line Items]
Revenue		332.5	335.5
Outcome-Based Solutions and Consulting | APME
Disaggregation of Revenue [Line Items]
Revenue		386.3	392.0
Outcome-Based Solutions and Consulting | Northern Europe And APME
Disaggregation of Revenue [Line Items]
Revenue		1,412.0	1,437.7
Permanent Recruitment | Americas
Disaggregation of Revenue [Line Items]
Revenue		179.9	257.6
Permanent Recruitment | Americas | United States
Disaggregation of Revenue [Line Items]
Revenue		133.0	209.0
Permanent Recruitment | Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue		46.9	48.6
Permanent Recruitment | Southern Europe
Disaggregation of Revenue [Line Items]
Revenue		172.8	171.6
Permanent Recruitment | Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue		58.9	57.9
Permanent Recruitment | Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue		55.0	51.7
Permanent Recruitment | Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue		58.9	62.0
Permanent Recruitment | Northern Europe
Disaggregation of Revenue [Line Items]
Revenue		153.2	179.3
Permanent Recruitment | APME
Disaggregation of Revenue [Line Items]
Revenue		109.4	149.1
Permanent Recruitment | Northern Europe And APME
Disaggregation of Revenue [Line Items]
Revenue		615.3	757.6
Other | Americas
Disaggregation of Revenue [Line Items]
Revenue		172.1	160.1
Other | Americas | United States
Disaggregation of Revenue [Line Items]
Revenue		162.6	153.5
Other | Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue		9.5	6.6
Other | Southern Europe
Disaggregation of Revenue [Line Items]
Revenue		125.0	70.6
Other | Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue		70.0	37.3
Other | Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue		26.8	22.6
Other | Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue		28.2	10.7
Other | Northern Europe
Disaggregation of Revenue [Line Items]
Revenue		102.1	86.6
Other | APME
Disaggregation of Revenue [Line Items]
Revenue		44.8	36.6
Other | Northern Europe And APME
Disaggregation of Revenue [Line Items]
Revenue		$ 444.0	$ 353.9

[1] (a) The United States revenues above represent revenues from our company-owned branches and franchise fees received from our franchise operations, which were $11.9, $12.8 and $12.8 for 2023, 2022, and 2021, respectively.