Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Disaggregation of Revenue [Line Items]
Revenue	$ 5,143.3	$ 4,924.4
Intercompany Eliminations
Disaggregation of Revenue [Line Items]
Revenue	14.5
Americas
Disaggregation of Revenue [Line Items]
Revenue	1,251.2	1,002.9
Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,213.8	984.9
Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	37.4	18.0
Americas | United States
Disaggregation of Revenue [Line Items]
Revenue	889.4	608.8
Americas | United States | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	858.8	594.7
Americas | United States | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	30.6	14.1
Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	361.8	394.1
Americas | Other Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	355.0	390.2
Americas | Other Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	6.8	3.9
Americas | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	1,076.8	885.0
Americas | Staffing and Interim | United States
Disaggregation of Revenue [Line Items]
Revenue	739.9	510.1
Americas | Staffing and Interim | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	336.9	374.9
Americas | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	69.1	38.7
Americas | Outcome-Based Solutions and Consulting | United States
Disaggregation of Revenue [Line Items]
Revenue	57.1	28.8
Americas | Outcome-Based Solutions and Consulting | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	12.0	9.9
Americas | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	65.4	34.6
Americas | Permanent Recruitment | United States
Disaggregation of Revenue [Line Items]
Revenue	54.2	28.5
Americas | Permanent Recruitment | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	11.2	6.1
Americas | Other
Disaggregation of Revenue [Line Items]
Revenue	39.9	44.6
Americas | Other | United States
Disaggregation of Revenue [Line Items]
Revenue	38.2	41.4
Americas | Other | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	1.7	3.2
Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	2,193.9	2,160.3
Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	2,152.7	2,129.3
Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	41.2	31.0
Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue	1,192.4	1,188.9
Southern Europe | France | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,177.0	1,176.9
Southern Europe | France | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	15.4	12.0
Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue	445.0	402.8
Southern Europe | Italy | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	432.4	393.5
Southern Europe | Italy | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	12.6	9.3
Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	556.5	568.6
Southern Europe | Other Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	543.3	558.9
Southern Europe | Other Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	13.2	9.7
Southern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	1,962.8	1,926.3
Southern Europe | Staffing and Interim | France
Disaggregation of Revenue [Line Items]
Revenue	1,101.5	1,084.0
Southern Europe | Staffing and Interim | Italy
Disaggregation of Revenue [Line Items]
Revenue	417.8	381.5
Southern Europe | Staffing and Interim | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	443.5	460.8
Southern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	172.5	186.8
Southern Europe | Outcome-Based Solutions and Consulting | France
Disaggregation of Revenue [Line Items]
Revenue	70.3	86.5
Southern Europe | Outcome-Based Solutions and Consulting | Italy
Disaggregation of Revenue [Line Items]
Revenue	8.1	7.7
Southern Europe | Outcome-Based Solutions and Consulting | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	94.1	92.6
Southern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	46.0	34.2
Southern Europe | Permanent Recruitment | France
Disaggregation of Revenue [Line Items]
Revenue	16.5	12.8
Southern Europe | Permanent Recruitment | Italy
Disaggregation of Revenue [Line Items]
Revenue	13.7	9.7
Southern Europe | Permanent Recruitment | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	15.8	11.7
Southern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue	12.6	13.0
Southern Europe | Other | France
Disaggregation of Revenue [Line Items]
Revenue	4.1	5.6
Southern Europe | Other | Italy
Disaggregation of Revenue [Line Items]
Revenue	5.4	3.9
Southern Europe | Other | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	3.1	3.5
Northern Europe
Disaggregation of Revenue [Line Items]
Revenue	1,094.5	1,133.8
Northern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,057.2	1,107.8
Northern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	37.3	26.0
Northern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	977.1	1,018.8
Northern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	55.0	64.8
Northern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	45.3	29.5
Northern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue	17.1	20.7
APME
Disaggregation of Revenue [Line Items]
Revenue	618.2	627.4
APME | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	597.1	607.9
APME | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	21.1	19.5
APME | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	463.1	480.8
APME | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	109.3	98.4
APME | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	37.2	37.4
APME | Other
Disaggregation of Revenue [Line Items]
Revenue	8.6	10.8
Northern Europe and APME
Disaggregation of Revenue [Line Items]
Revenue	5,157.8	4,924.4
Reportable Segments
Disaggregation of Revenue [Line Items]
Revenue	5,157.8	4,924.4
Reportable Segments | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	5,020.8	4,829.9
Reportable Segments | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	137.0	94.5
Reportable Segments | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	4,479.8	4,310.9
Reportable Segments | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	405.9	388.7
Reportable Segments | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	193.9	135.7
Reportable Segments | Other
Disaggregation of Revenue [Line Items]
Revenue	$ 78.2	$ 89.1