Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended				9 Months Ended				12 Months Ended
		Sep. 30, 2023		Sep. 30, 2022		Sep. 30, 2023		Sep. 30, 2022		Dec. 31, 2022
Disaggregation of Revenue [Line Items]
Revenue	[2]	$ 4,675.6	[1]	$ 4,800.9	[1]	$ 14,284.0	[3]	$ 15,018.3	[3]
Intercompany Eliminations
Disaggregation of Revenue [Line Items]
Revenue		(23.4)	[1]	(19.7)	[1]	(64.1)	[3]	(55.0)	[3]
Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue						90.0
Outcome-Based Solutions and Consulting | United States
Disaggregation of Revenue [Line Items]
Revenue						171.6				$ 58.4
Americas | Reportable subsegments
Disaggregation of Revenue [Line Items]
Revenue		1,111.3	[1]	1,239.8	[1]	3,340.2	[3]	3,753.7	[3]
Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		1,083.8		1,200.8		3,256.9		3,636.3
Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		27.5		39.0		83.3		117.4
Americas | United States
Disaggregation of Revenue [Line Items]
Revenue		752.6		886.6		2,259.3		2,679.9
Americas | United States | Reportable subsegments
Disaggregation of Revenue [Line Items]
Revenue	[4]	752.6	[1]	886.6	[1]	2,259.3	[3]	2,679.9	[3]
Americas | United States | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		732.9		855.7		2,198.6		2,585.5
Americas | United States | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		19.7		30.9		60.7		94.4
Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue		358.7		353.2		1,080.9		1,073.8
Americas | Other Americas | Reportable subsegments
Disaggregation of Revenue [Line Items]
Revenue		358.7	[1]	353.2	[1]	1,080.9	[3]	1,073.8	[3]
Americas | Other Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		350.9		345.1		1,058.3		1,050.8
Americas | Other Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		7.8		8.1		22.6		23.0
Americas | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue		4,111.4	[1]	4,212.6	[1]	3,023.5	[3]	3,378.7	[3]
Americas | Staffing and Interim | United States
Disaggregation of Revenue [Line Items]
Revenue		673.6	[1]	788.6	[1]	2,018.9	[3]	2,384.4	[3]
Americas | Staffing and Interim | Other Americas
Disaggregation of Revenue [Line Items]
Revenue		332.7	[1]	325.5	[1]	1,004.6	[3]	994.3	[3]
Americas | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue		338.3	[1]	332.7	[1]	48.6	[3]	53.5	[3]
Americas | Outcome-Based Solutions and Consulting | United States
Disaggregation of Revenue [Line Items]
Revenue		5.3	[1]	6.2	[1]	14.5	[3]	16.0	[3]
Americas | Outcome-Based Solutions and Consulting | Other Americas
Disaggregation of Revenue [Line Items]
Revenue		11.5	[1]	13.0	[1]	34.1	[3]	37.5	[3]
Americas | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue		142.8	[1]	187.6	[1]	138.9	[3]	203.7	[3]
Americas | Permanent Recruitment | United States
Disaggregation of Revenue [Line Items]
Revenue		33.7	[1]	54.4	[1]	103.5	[3]	166.6	[3]
Americas | Permanent Recruitment | Other Americas
Disaggregation of Revenue [Line Items]
Revenue		12.0	[1]	13.0	[1]	35.4	[3]	37.1	[3]
Americas | Other
Disaggregation of Revenue [Line Items]
Revenue		106.5	[1]	87.7	[1]	129.2	[3]	117.8	[3]
Americas | Other | United States
Disaggregation of Revenue [Line Items]
Revenue		40.0	[1]	37.4	[1]	122.4	[3]	112.9	[3]
Americas | Other | Other Americas
Disaggregation of Revenue [Line Items]
Revenue		2.5	[1]	1.7	[1]	6.8	[3]	4.9	[3]
Southern Europe | Reportable subsegments
Disaggregation of Revenue [Line Items]
Revenue		2,108.7	[1]	2,039.8	[1]	6,403.5	[3]	6,435.1	[3]
Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		2,074.4		2,005.7		6,285.3		6,318.6
Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		34.3		34.1		118.2		116.5
Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue		1,209.9		1,159.5		3,657.4		3,590.1
Southern Europe | France | Reportable subsegments
Disaggregation of Revenue [Line Items]
Revenue		1,209.9	[1]	1,159.5	[1]	3,657.4	[3]	3,590.1	[3]
Southern Europe | France | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		1,198.6		1,147.8		3,616.2		3,548.2
Southern Europe | France | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		11.3		11.7		41.2		41.9
Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue		413.7		395.1		1,293.7		1,294.4
Southern Europe | Italy | Reportable subsegments
Disaggregation of Revenue [Line Items]
Revenue		413.7	[1]	395.1	[1]	1,293.7	[3]	1,294.4	[3]
Southern Europe | Italy | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		402.7		385.0		1,254.4		1,258.3
Southern Europe | Italy | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		11.0		10.1		39.3		36.1
Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue		485.1		485.2		1,452.4		1,550.6
Southern Europe | Other Southern Europe | Reportable subsegments
Disaggregation of Revenue [Line Items]
Revenue		485.1	[1]	485.2	[1]	1,452.4	[3]	1,550.6	[3]
Southern Europe | Other Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		473.1		472.9		1,414.7		1,512.1
Southern Europe | Other Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		12.0		12.3		37.7		38.5
Southern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue		1,006.3	[1]	1,114.1	[1]	5,709.0	[3]	5,775.0	[3]
Southern Europe | Staffing and Interim | France
Disaggregation of Revenue [Line Items]
Revenue		1,113.8	[1]	1,071.8	[1]	3,348.3	[3]	3,323.2	[3]
Southern Europe | Staffing and Interim | Italy
Disaggregation of Revenue [Line Items]
Revenue		388.8	[1]	372.8	[1]	1,207.1	[3]	1,214.5	[3]
Southern Europe | Staffing and Interim | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue		390.4	[1]	390.8	[1]	1,153.6	[3]	1,237.3	[3]
Southern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue		16.8	[1]	19.2	[1]	471.3	[3]	482.8	[3]
Southern Europe | Outcome-Based Solutions and Consulting | France
Disaggregation of Revenue [Line Items]
Revenue		67.9	[1]	61.9	[1]	212.3	[3]	200.5	[3]
Southern Europe | Outcome-Based Solutions and Consulting | Italy
Disaggregation of Revenue [Line Items]
Revenue		8.0	[1]	6.7	[1]	27.6	[3]	24.0	[3]
Southern Europe | Outcome-Based Solutions and Consulting | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue		73.5	[1]	77.0	[1]	231.4	[3]	258.3	[3]
Southern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue		45.7	[1]	67.4	[1]	133.6	[3]	132.1	[3]
Southern Europe | Permanent Recruitment | France
Disaggregation of Revenue [Line Items]
Revenue		12.7	[1]	12.9	[1]	45.4	[3]	45.8	[3]
Southern Europe | Permanent Recruitment | Italy
Disaggregation of Revenue [Line Items]
Revenue		12.0	[1]	11.1	[1]	42.3	[3]	39.3	[3]
Southern Europe | Permanent Recruitment | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue		14.5	[1]	15.3	[1]	45.9	[3]	47.0	[3]
Southern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue		42.5	[1]	39.1	[1]	89.6	[3]	45.2	[3]
Southern Europe | Other | France
Disaggregation of Revenue [Line Items]
Revenue		15.5	[1]	12.9	[1]	51.4	[3]	20.6	[3]
Southern Europe | Other | Italy
Disaggregation of Revenue [Line Items]
Revenue		4.9	[1]	4.5	[1]	16.7	[3]	16.6	[3]
Southern Europe | Other | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue		6.7	[1]	2.1	[1]	21.5	[3]	8.0	[3]
Northern Europe | Reportable subsegments
Disaggregation of Revenue [Line Items]
Revenue		914.2	[1]	954.1	[1]	2,834.3	[3]	3,075.7	[3]
Northern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		885.7		919.4		2,740.1		2,965.3
Northern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		28.5		34.7		94.2		110.4
Northern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue		773.8	[1]	815.3	[1]	2,389.2	[3]	2,627.4	[3]	$ 24.8
Northern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue		78.4	[1]	76.1	[1]	250.7	[3]	249.0	[3]
Northern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue		36.5	[1]	42.9	[1]	119.5	[3]	135.2	[3]
Northern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue		25.5	[1]	19.8	[1]	74.9	[3]	64.1	[3]
APME | Reportable subsegments
Disaggregation of Revenue [Line Items]
Revenue		564.8	[1]	586.9	[1]	1,770.1	[3]	1,808.8	[3]
APME | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		550.3		565.3		1,713.3		1,742.9
APME | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		14.5		21.6		56.8		65.9
APME | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue		438.3	[1]	447.8	[1]	1,346.4	[3]	1,366.5	[3]
APME | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue		93.7	[1]	91.8	[1]	292.1	[3]	299.9	[3]
APME | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue		21.4	[1]	38.0	[1]	96.9	[3]	115.7	[3]
APME | Other
Disaggregation of Revenue [Line Items]
Revenue		11.4	[1]	9.3	[1]	34.7	[3]	26.7	[3]
Northern Europe and APME | Reportable subsegments
Disaggregation of Revenue [Line Items]
Revenue		4,699.0	[1]	4,820.6	[1]	14,348.1	[3]	15,073.3	[3]
Northern Europe and APME | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue		4,594.2		4,691.2		13,995.6		14,663.1
Northern Europe and APME | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue		104.8		129.4		352.5		410.2
Northern Europe and APME | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue		1,893.0	[1]	1,835.4	[1]	12,468.1	[3]	13,147.6	[3]
Northern Europe and APME | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue		149.4	[1]	145.6	[1]	1,062.7	[3]	1,085.2	[3]
Northern Europe and APME | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue		39.2	[1]	39.3	[1]	488.9	[3]	586.7	[3]
Northern Europe and APME | Other
Disaggregation of Revenue [Line Items]
Revenue		$ 27.1	[1]	$ 19.5	[1]	$ 328.4	[3]	$ 253.8	[3]

[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.

[4] In the United States, revenues from services included fees received from the related franchise offices of $3.1 and $3.7 for the three months ended September 30, 2023 and 2022, respectively, and $8.9 and $9.7 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 $98.6 and $121.9 for the three months ended September 30, 2023 and 2022, respectively, and $298.7 and $360.8 for the nine months ended September 30, 2023 and 2022, respectively.