Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 4,924.4	$ 4,619.1
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	4,829.9	4,524.6
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	94.5	94.5
Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	4,310.9	4,035.7
Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	388.7	374.0
Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	135.7	133.3
Other
Disaggregation of Revenue [Line Items]
Revenue	89.1	76.1
Americas
Disaggregation of Revenue [Line Items]
Revenue	1,002.9	1,011.0
Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	984.9	994.3
Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	18.0	16.7
Americas | United States
Disaggregation of Revenue [Line Items]
Revenue	608.8	610.9
Americas | United States | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	594.7	597.8
Americas | United States | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	14.1	13.1
Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	394.1	400.1
Americas | Other Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	390.2	396.5
Americas | Other Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	3.9	3.6
Americas | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	885.0	898.2
Americas | Staffing and Interim | United States
Disaggregation of Revenue [Line Items]
Revenue	510.1	518.7
Americas | Staffing and Interim | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	374.9	379.5
Americas | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	38.7	42.5
Americas | Outcome-Based Solutions and Consulting | United States
Disaggregation of Revenue [Line Items]
Revenue	28.8	30.1
Americas | Outcome-Based Solutions and Consulting | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	9.9	12.4
Americas | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	34.6	30.5
Americas | Permanent Recruitment | United States
Disaggregation of Revenue [Line Items]
Revenue	28.5	25.0
Americas | Permanent Recruitment | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	6.1	5.5
Americas | Other
Disaggregation of Revenue [Line Items]
Revenue	44.6	39.8
Americas | Other | United States
Disaggregation of Revenue [Line Items]
Revenue	41.4	37.1
Americas | Other | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	3.2	2.7
Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	2,160.3	1,944.7
Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	2,129.3	1,913.6
Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	31.0	31.1
Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue	1,188.9	1,093.8
Southern Europe | France | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,176.9	1,082.1
Southern Europe | France | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	12.0	11.7
Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue	402.8	327.7
Southern Europe | Italy | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	393.5	320.0
Southern Europe | Italy | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	9.3	7.7
Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	568.6	523.2
Southern Europe | Other Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	558.9	511.5
Southern Europe | Other Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	9.7	11.7
Southern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	1,926.3	1,721.3
Southern Europe | Staffing and Interim | France
Disaggregation of Revenue [Line Items]
Revenue	1,084.0	998.3
Southern Europe | Staffing and Interim | Italy
Disaggregation of Revenue [Line Items]
Revenue	381.5	307.1
Southern Europe | Staffing and Interim | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	460.8	415.9
Southern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	186.8	177.4
Southern Europe | Outcome-Based Solutions and Consulting | France
Disaggregation of Revenue [Line Items]
Revenue	86.5	78.1
Southern Europe | Outcome-Based Solutions and Consulting | Italy
Disaggregation of Revenue [Line Items]
Revenue	7.7	8.7
Southern Europe | Outcome-Based Solutions and Consulting | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	92.6	90.6
Southern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	34.2	34.6
Southern Europe | Permanent Recruitment | France
Disaggregation of Revenue [Line Items]
Revenue	12.8	12.5
Southern Europe | Permanent Recruitment | Italy
Disaggregation of Revenue [Line Items]
Revenue	9.7	8.2
Southern Europe | Permanent Recruitment | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	11.7	13.9
Southern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue	13.0	11.4
Southern Europe | Other | France
Disaggregation of Revenue [Line Items]
Revenue	5.6	4.9
Southern Europe | Other | Italy
Disaggregation of Revenue [Line Items]
Revenue	3.9	3.7
Southern Europe | Other | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	3.5	2.8
Northern Europe
Disaggregation of Revenue [Line Items]
Revenue	1,133.8	1,068.5
Northern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,107.8	1,041.4
Northern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	26.0	27.1
Northern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	1,018.8	949.6
Northern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	64.8	70.9
Northern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	29.5	31.3
Northern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue	20.7	16.7
APME
Disaggregation of Revenue [Line Items]
Revenue	627.4	594.9
APME | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	607.9	575.3
APME | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	19.5	19.6
APME | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	480.8	466.6
APME | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	98.4	83.2
APME | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	37.4	36.9
APME | Other
Disaggregation of Revenue [Line Items]
Revenue	$ 10.8	$ 8.2