Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2023		Mar. 31, 2022	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
Revenue	$ 4,752.3		$ 5,143.3
Intercompany Eliminations
Disaggregation of Revenue [Line Items]
Revenue	(19.3)		(14.5)
Outcome-Based Solutions and Consulting | United States
Disaggregation of Revenue [Line Items]
Revenue				$ 52.8
Americas
Disaggregation of Revenue [Line Items]
Revenue	1,130.2		1,251.2
Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,101.3		1,213.8
Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	28.9		37.4
Americas | United States
Disaggregation of Revenue [Line Items]
Revenue	770.0		889.4
Americas | United States | Reportable Subsegments [Member]
Disaggregation of Revenue [Line Items]
Revenue	770.0		889.4
Americas | United States | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	748.4		858.8
Americas | United States | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	21.6		30.6
Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	360.2		361.8
Americas | Other Americas | Reportable Subsegments [Member]
Disaggregation of Revenue [Line Items]
Revenue	360.2		361.8
Americas | Other Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	352.9		355.0
Americas | Other Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	7.3		6.8
Americas | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	1,022.7		1,129.6
Americas | Staffing and Interim | United States
Disaggregation of Revenue [Line Items]
Revenue	687.1		792.7
Americas | Staffing and Interim | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	335.6		336.9
Americas | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	15.3		16.3
Americas | Outcome-Based Solutions and Consulting | United States
Disaggregation of Revenue [Line Items]
Revenue	4.2	[1]	4.3
Americas | Outcome-Based Solutions and Consulting | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	11.1		12.0
Americas | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	49.5		65.4
Americas | Permanent Recruitment | United States
Disaggregation of Revenue [Line Items]
Revenue	37.8		54.2
Americas | Permanent Recruitment | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	11.7		11.2
Americas | Other
Disaggregation of Revenue [Line Items]
Revenue	42.7		39.9
Americas | Other | United States
Disaggregation of Revenue [Line Items]
Revenue	40.9		38.2
Americas | Other | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	1.8		1.7
Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	2,067.9		2,193.9
Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	2,025.6		2,152.7
Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	42.3		41.2
Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue	1,169.3		1,192.4
Southern Europe | France | Reportable Subsegments [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,169.3		1,192.4
Southern Europe | France | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,154.1		1,177.0
Southern Europe | France | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	15.2		15.4
Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue	422.2		445.0
Southern Europe | Italy | Reportable Subsegments [Member]
Disaggregation of Revenue [Line Items]
Revenue	422.2		445.0
Southern Europe | Italy | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	408.2		432.4
Southern Europe | Italy | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	14.0		12.6
Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	476.4		556.5
Southern Europe | Other Southern Europe | Reportable Subsegments [Member]
Disaggregation of Revenue [Line Items]
Revenue	476.4		556.5
Southern Europe | Other Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	463.3		543.3
Southern Europe | Other Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	13.1		13.2
Southern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	1,827.0		1,962.8
Southern Europe | Staffing and Interim | France
Disaggregation of Revenue [Line Items]
Revenue	1,061.7		1,101.5
Southern Europe | Staffing and Interim | Italy
Disaggregation of Revenue [Line Items]
Revenue	392.7		417.8
Southern Europe | Staffing and Interim | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	372.6		443.5
Southern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	163.2		172.5
Southern Europe | Outcome-Based Solutions and Consulting | France
Disaggregation of Revenue [Line Items]
Revenue	73.2		70.3
Southern Europe | Outcome-Based Solutions and Consulting | Italy
Disaggregation of Revenue [Line Items]
Revenue	9.4		8.1
Southern Europe | Outcome-Based Solutions and Consulting | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	80.6		94.1
Southern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	47.4		46.0
Southern Europe | Permanent Recruitment | France
Disaggregation of Revenue [Line Items]
Revenue	16.5		16.5
Southern Europe | Permanent Recruitment | Italy
Disaggregation of Revenue [Line Items]
Revenue	15.0		13.7
Southern Europe | Permanent Recruitment | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	15.9		15.8
Southern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue	30.3		12.6
Southern Europe | Other | France
Disaggregation of Revenue [Line Items]
Revenue	17.9		4.1
Southern Europe | Other | Italy
Disaggregation of Revenue [Line Items]
Revenue	5.1		5.4
Southern Europe | Other | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	7.3		3.1
Northern Europe
Disaggregation of Revenue [Line Items]
Revenue	967.6		1,094.5
Northern Europe | Reportable Subsegments [Member]
Disaggregation of Revenue [Line Items]
Revenue	967.6		1,094.5
Northern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	933.1		1,057.2
Northern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	34.5		37.3
Northern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	810.6		938.1	$ 39.0
Northern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	89.4		94.0
Northern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	43.4		45.3
Northern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue	24.2		17.1
APME
Disaggregation of Revenue [Line Items]
Revenue	605.9		618.2
APME | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	584.6		597.1
APME | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	21.3		21.1
APME | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	454.9		463.1
APME | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	100.9		109.3
APME | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	39.0		37.2
APME | Other
Disaggregation of Revenue [Line Items]
Revenue	11.1		8.6
Northern Europe and APME
Disaggregation of Revenue [Line Items]
Revenue	4,771.6		5,157.8
Northern Europe and APME | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	4,644.6		5,020.8
Northern Europe and APME | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	127.0		137.0
Northern Europe and APME | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	4,115.2		4,493.6
Northern Europe and APME | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	368.8		392.1
Northern Europe and APME | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	179.3		193.9
Northern Europe and APME | Other
Disaggregation of Revenue [Line Items]
Revenue	$ 108.3		$ 78.2

[1] (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, $52.8 was reclassified from Outcome-Based Solutions and Consulting to Staffing and Interim. In Northern Europe, $39.0 was reclassified from Staffing and Interim to Outcome-Based Solutions and Consulting.