Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 20,724.4	$ 18,001.0	$ 20,863.5
Intercompany Eliminations
Disaggregation of Revenue [Line Items]
Revenue	(37.7)	0.0	0.0
United States
Disaggregation of Revenue [Line Items]
Revenue		2,327.2
Americas
Disaggregation of Revenue [Line Items]
Revenue	4,263.7	3,792.4	4,278.6
Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	4,162.5	3,735.3
Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	101.2	57.1
Americas | United States
Disaggregation of Revenue [Line Items]
Revenue	2,743.3	2,327.2
Americas | United States | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	2,663.9	2,281.7
Americas | United States | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	79.4	45.5
Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	1,520.4	1,465.2
Americas | Other Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,498.6	1,453.6
Americas | Other Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	21.8	11.6
Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	9,346.8	7,855.2	9,194.4
Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	9,212.3	7,760.2
Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	134.5	95.0
Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue	5,171.3	4,338.1
Southern Europe | France | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	5,122.4	4,301.8
Southern Europe | France | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	48.9	36.3
Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue	1,795.4	1,370.7
Southern Europe | Italy | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,752.8	1,345.4
Southern Europe | Italy | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	42.6	25.3
Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	2,380.1	2,146.4
Southern Europe | Other Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	2,337.1	2,113.0
Southern Europe | Other Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	43.0	33.4
Northern Europe
Disaggregation of Revenue [Line Items]
Revenue	4,670.5	3,976.7	4,735.5
Northern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	4,545.0	3,889.2
Northern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	125.5	87.5
APME
Disaggregation of Revenue [Line Items]
Revenue	2,481.1	2,376.7	2,655.0
APME | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	2,401.1	2,310.0
APME | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	80.0	66.7
Northern Europe And APME
Disaggregation of Revenue [Line Items]
Revenue	20,762.1	18,001.0	$ 20,863.5
Northern Europe And APME | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	20,320.9	17,694.7
Northern Europe And APME | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	441.2	306.3
Staffing and Interim | Americas
Disaggregation of Revenue [Line Items]
Revenue	3,722.4	3,335.9
Staffing and Interim | Americas | United States
Disaggregation of Revenue [Line Items]
Revenue	2,292.1	1,942.9
Staffing and Interim | Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	1,430.3	1,393.0
Staffing and Interim | Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	8,405.7	7,034.3
Staffing and Interim | Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue	4,765.1	3,985.6
Staffing and Interim | Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue	1,695.6	1,296.7
Staffing and Interim | Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	1,945.0	1,752.0
Staffing and Interim | Northern Europe
Disaggregation of Revenue [Line Items]
Revenue	4,191.4	3,545.7
Staffing and Interim | APME
Disaggregation of Revenue [Line Items]
Revenue	1,886.6	1,876.3
Staffing and Interim | Northern Europe And APME
Disaggregation of Revenue [Line Items]
Revenue	18,206.1	15,792.2
Outcome-Based Solutions and Consulting | Americas
Disaggregation of Revenue [Line Items]
Revenue	200.8	167.1
Outcome-Based Solutions and Consulting | Americas | United States
Disaggregation of Revenue [Line Items]
Revenue	154.9	125.3
Outcome-Based Solutions and Consulting | Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	45.9	41.8
Outcome-Based Solutions and Consulting | Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	733.1	667.3
Outcome-Based Solutions and Consulting | Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue	329.8	295.7
Outcome-Based Solutions and Consulting | Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue	33.9	30.1
Outcome-Based Solutions and Consulting | Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	369.4	341.5
Outcome-Based Solutions and Consulting | Northern Europe
Disaggregation of Revenue [Line Items]
Revenue	255.5	264.9
Outcome-Based Solutions and Consulting | APME
Disaggregation of Revenue [Line Items]
Revenue	408.1	340.7
Outcome-Based Solutions and Consulting | Northern Europe And APME
Disaggregation of Revenue [Line Items]
Revenue	1,597.5	1,440.0
Permanent Recruitment | Americas
Disaggregation of Revenue [Line Items]
Revenue	182.3	106.2
Permanent Recruitment | Americas | United States
Disaggregation of Revenue [Line Items]
Revenue	147.8	88.4
Permanent Recruitment | Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	34.5	17.8
Permanent Recruitment | Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	151.7	106.6
Permanent Recruitment | Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue	53.5	39.7
Permanent Recruitment | Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue	45.6	27.0
Permanent Recruitment | Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	52.6	39.9
Permanent Recruitment | Northern Europe
Disaggregation of Revenue [Line Items]
Revenue	145.9	100.3
Permanent Recruitment | APME
Disaggregation of Revenue [Line Items]
Revenue	143.2	121.6
Permanent Recruitment | Northern Europe And APME
Disaggregation of Revenue [Line Items]
Revenue	623.1	434.7
Other | Americas
Disaggregation of Revenue [Line Items]
Revenue	158.2	183.2
Other | Americas | United States
Disaggregation of Revenue [Line Items]
Revenue	148.5	170.6
Other | Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	9.7	12.6
Other | Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	56.3	47.0
Other | Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue	22.9	17.1
Other | Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue	20.3	16.9
Other | Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	13.1	13.0
Other | Northern Europe
Disaggregation of Revenue [Line Items]
Revenue	77.7	65.8
Other | APME
Disaggregation of Revenue [Line Items]
Revenue	43.2	38.1
Other | Northern Europe And APME
Disaggregation of Revenue [Line Items]
Revenue	$ 335.4	$ 334.1