Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 5,054.9	$ 4,584.8	$ 3,742.2	$ 4,619.1	$ 5,196.6	$ 5,248.9	$ 5,373.1	$ 5,044.9	$ 18,001.0	$ 20,863.5	$ 21,991.2
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue									17,694.7	20,432.1
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue									306.3	431.4
Americas
Disaggregation of Revenue [Line Items]
Revenue									3,792.4	4,278.6	4,258.9
Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue									3,735.3	4,207.3
Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue									57.1	71.3
Americas | United States
Disaggregation of Revenue [Line Items]
Revenue									2,327.2	2,590.6	2,608.9
Americas | United States | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue									2,281.7	2,536.7
Americas | United States | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue									45.5	53.9
Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue									1,465.2	1,688.0
Americas | Other Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue									1,453.6	1,670.6
Americas | Other Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue									11.6	17.4
Southern Europe
Disaggregation of Revenue [Line Items]
Revenue									7,855.2	9,194.4	9,399.9
Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue									7,760.2	9,062.0
Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue									95.0	132.4
Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue									4,338.1	5,479.6
Southern Europe | France | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue									4,301.8	5,430.7
Southern Europe | France | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue									36.3	48.9
Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue									1,370.7	1,508.3
Southern Europe | Italy | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue									1,345.4	1,473.6
Southern Europe | Italy | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue									25.3	34.7
Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue									2,146.4	2,206.5
Southern Europe | Other Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue									2,113.0	2,157.7
Southern Europe | Other Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue									33.4	48.8
Northern Europe
Disaggregation of Revenue [Line Items]
Revenue									3,976.7	4,735.5	5,417.5
Northern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue									3,889.2	4,608.7
Northern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue									87.5	126.8
APME
Disaggregation of Revenue [Line Items]
Revenue									2,376.7	2,655.0	$ 2,914.9
APME | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue									2,310.0	2,554.1
APME | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue									66.7	100.9
Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue									15,792.2	18,486.1
Staffing and Interim | Americas
Disaggregation of Revenue [Line Items]
Revenue									3,335.9	3,783.4
Staffing and Interim | Americas | United States
Disaggregation of Revenue [Line Items]
Revenue									1,942.9	2,186.8
Staffing and Interim | Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue									1,393.0	1,596.6
Staffing and Interim | Southern Europe
Disaggregation of Revenue [Line Items]
Revenue									7,034.3	8,362.1
Staffing and Interim | Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue									3,985.6	5,159.5
Staffing and Interim | Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue									1,296.7	1,401.9
Staffing and Interim | Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue									1,752.0	1,800.7
Staffing and Interim | Northern Europe
Disaggregation of Revenue [Line Items]
Revenue									3,545.7	4,180.9
Staffing and Interim | APME
Disaggregation of Revenue [Line Items]
Revenue									1,876.3	2,159.7
Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue									1,440.0	1,462.4
Outcome-Based Solutions and Consulting | Americas
Disaggregation of Revenue [Line Items]
Revenue									167.1	203.7
Outcome-Based Solutions and Consulting | Americas | United States
Disaggregation of Revenue [Line Items]
Revenue									125.3	150.7
Outcome-Based Solutions and Consulting | Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue									41.8	53.0
Outcome-Based Solutions and Consulting | Southern Europe
Disaggregation of Revenue [Line Items]
Revenue									667.3	617.1
Outcome-Based Solutions and Consulting | Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue									295.7	237.8
Outcome-Based Solutions and Consulting | Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue									30.1	45.5
Outcome-Based Solutions and Consulting | Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue									341.5	333.8
Outcome-Based Solutions and Consulting | Northern Europe
Disaggregation of Revenue [Line Items]
Revenue									264.9	339.9
Outcome-Based Solutions and Consulting | APME
Disaggregation of Revenue [Line Items]
Revenue									340.7	301.7
Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue									434.7	577.5
Permanent Recruitment | Americas
Disaggregation of Revenue [Line Items]
Revenue									106.2	123.4
Permanent Recruitment | Americas | United States
Disaggregation of Revenue [Line Items]
Revenue									88.4	98.0
Permanent Recruitment | Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue									17.8	25.4
Permanent Recruitment | Southern Europe
Disaggregation of Revenue [Line Items]
Revenue									106.6	148.6
Permanent Recruitment | Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue									39.7	52.1
Permanent Recruitment | Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue									27.0	37.6
Permanent Recruitment | Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue									39.9	58.9
Permanent Recruitment | Northern Europe
Disaggregation of Revenue [Line Items]
Revenue									100.3	145.6
Permanent Recruitment | APME
Disaggregation of Revenue [Line Items]
Revenue									121.6	159.9
Other
Disaggregation of Revenue [Line Items]
Revenue									334.1	337.5
Other | Americas
Disaggregation of Revenue [Line Items]
Revenue									183.2	168.1
Other | Americas | United States
Disaggregation of Revenue [Line Items]
Revenue									170.6	155.1
Other | Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue									12.6	13.0
Other | Southern Europe
Disaggregation of Revenue [Line Items]
Revenue									47.0	66.6
Other | Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue									17.1	30.2
Other | Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue									16.9	23.3
Other | Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue									13.0	13.1
Other | Northern Europe
Disaggregation of Revenue [Line Items]
Revenue									65.8	69.1
Other | APME
Disaggregation of Revenue [Line Items]
Revenue									$ 38.1	$ 33.7