Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation Of Revenue [Line Items]
Revenue	$ 5,196.6	$ 5,248.9	$ 5,373.1	$ 5,044.9	$ 5,393.2	$ 5,418.7	$ 5,656.9	$ 5,522.4	$ 20,863.5	$ 21,991.2	$ 21,034.3
Services transferred over time
Disaggregation Of Revenue [Line Items]
Revenue									20,432.1	21,517.5
Services transferred at a point in time
Disaggregation Of Revenue [Line Items]
Revenue									431.4	473.7
Americas
Disaggregation Of Revenue [Line Items]
Revenue									4,182.3	4,159.3	4,216.4
Americas | Services transferred over time
Disaggregation Of Revenue [Line Items]
Revenue									4,111.0	4,090.7
Americas | Services transferred at a point in time
Disaggregation Of Revenue [Line Items]
Revenue									71.3	68.6
Americas | United States
Disaggregation Of Revenue [Line Items]
Revenue									2,507.0	2,522.3	2,659.0
Americas | United States | Services transferred over time
Disaggregation Of Revenue [Line Items]
Revenue									2,453.1	2,471.0
Americas | United States | Services transferred at a point in time
Disaggregation Of Revenue [Line Items]
Revenue									53.9	51.3
Americas | Other Americas
Disaggregation Of Revenue [Line Items]
Revenue									1,675.3	1,637.0
Americas | Other Americas | Services transferred over time
Disaggregation Of Revenue [Line Items]
Revenue									1,657.9	1,619.7
Americas | Other Americas | Services transferred at a point in time
Disaggregation Of Revenue [Line Items]
Revenue									17.4	17.3
Southern Europe
Disaggregation Of Revenue [Line Items]
Revenue									9,166.4	9,371.6	8,657.0
Southern Europe | Services transferred over time
Disaggregation Of Revenue [Line Items]
Revenue									9,034.0	9,239.9
Southern Europe | Services transferred at a point in time
Disaggregation Of Revenue [Line Items]
Revenue									132.4	131.7
Southern Europe | France
Disaggregation Of Revenue [Line Items]
Revenue									5,459.7	5,827.7
Southern Europe | France | Services transferred over time
Disaggregation Of Revenue [Line Items]
Revenue									5,410.9	5,775.1
Southern Europe | France | Services transferred at a point in time
Disaggregation Of Revenue [Line Items]
Revenue									48.8	52.6
Southern Europe | Italy
Disaggregation Of Revenue [Line Items]
Revenue									1,506.5	1,670.6
Southern Europe | Italy | Services transferred over time
Disaggregation Of Revenue [Line Items]
Revenue									1,471.8	1,637.1
Southern Europe | Italy | Services transferred at a point in time
Disaggregation Of Revenue [Line Items]
Revenue									34.7	33.5
Southern Europe | Other Southern Europe
Disaggregation Of Revenue [Line Items]
Revenue									2,200.2	1,873.3
Southern Europe | Other Southern Europe | Services transferred over time
Disaggregation Of Revenue [Line Items]
Revenue									2,151.3	1,827.7
Southern Europe | Other Southern Europe | Services transferred at a point in time
Disaggregation Of Revenue [Line Items]
Revenue									48.9	45.6
Northern Europe
Disaggregation Of Revenue [Line Items]
Revenue									4,691.3	5,370.5	5,306.4
Northern Europe | Services transferred over time
Disaggregation Of Revenue [Line Items]
Revenue									4,564.5	5,229.1
Northern Europe | Services transferred at a point in time
Disaggregation Of Revenue [Line Items]
Revenue									126.8	141.4
APME
Disaggregation Of Revenue [Line Items]
Revenue									2,627.2	2,890.3	2,636.4
APME | Services transferred over time
Disaggregation Of Revenue [Line Items]
Revenue									2,526.3	2,758.3
APME | Services transferred at a point in time
Disaggregation Of Revenue [Line Items]
Revenue									100.9	132.0
Right Management
Disaggregation Of Revenue [Line Items]
Revenue									196.3	199.5	$ 218.1
Right Management | Services transferred over time
Disaggregation Of Revenue [Line Items]
Revenue									196.3	199.5
Right Management | Services transferred at a point in time
Disaggregation Of Revenue [Line Items]
Revenue									0.0	0.0
Staffing and Interim
Disaggregation Of Revenue [Line Items]
Revenue									18,486.1	19,498.2
Staffing and Interim | Americas
Disaggregation Of Revenue [Line Items]
Revenue									3,783.4	3,769.4
Staffing and Interim | Americas | United States
Disaggregation Of Revenue [Line Items]
Revenue									2,186.8	2,208.3
Staffing and Interim | Americas | Other Americas
Disaggregation Of Revenue [Line Items]
Revenue									1,596.6	1,561.1
Staffing and Interim | Southern Europe
Disaggregation Of Revenue [Line Items]
Revenue									8,362.1	8,588.5
Staffing and Interim | Southern Europe | France
Disaggregation Of Revenue [Line Items]
Revenue									5,159.5	5,526.5
Staffing and Interim | Southern Europe | Italy
Disaggregation Of Revenue [Line Items]
Revenue									1,401.9	1,565.6
Staffing and Interim | Southern Europe | Other Southern Europe
Disaggregation Of Revenue [Line Items]
Revenue									1,800.7	1,496.4
Staffing and Interim | Northern Europe
Disaggregation Of Revenue [Line Items]
Revenue									4,180.9	4,746.3
Staffing and Interim | APME
Disaggregation Of Revenue [Line Items]
Revenue									2,159.7	2,394.0
Staffing and Interim | Right Management
Disaggregation Of Revenue [Line Items]
Revenue									0.0	0.0
Outcome-Based Solutions and Consulting
Disaggregation Of Revenue [Line Items]
Revenue									1,462.4	1,517.2
Outcome-Based Solutions and Consulting | Americas
Disaggregation Of Revenue [Line Items]
Revenue									179.9	176.9
Outcome-Based Solutions and Consulting | Americas | United States
Disaggregation Of Revenue [Line Items]
Revenue									130.9	129.7
Outcome-Based Solutions and Consulting | Americas | Other Americas
Disaggregation Of Revenue [Line Items]
Revenue									49.0	47.2
Outcome-Based Solutions and Consulting | Southern Europe
Disaggregation Of Revenue [Line Items]
Revenue									609.3	579.3
Outcome-Based Solutions and Consulting | Southern Europe | France
Disaggregation Of Revenue [Line Items]
Revenue									232.4	224.7
Outcome-Based Solutions and Consulting | Southern Europe | Italy
Disaggregation Of Revenue [Line Items]
Revenue									44.6	48.4
Outcome-Based Solutions and Consulting | Southern Europe | Other Southern Europe
Disaggregation Of Revenue [Line Items]
Revenue									332.3	306.2
Outcome-Based Solutions and Consulting | Northern Europe
Disaggregation Of Revenue [Line Items]
Revenue									332.3	426.2
Outcome-Based Solutions and Consulting | APME
Disaggregation Of Revenue [Line Items]
Revenue									294.7	284.9
Outcome-Based Solutions and Consulting | Right Management
Disaggregation Of Revenue [Line Items]
Revenue									46.2	49.9
Permanent Recruitment
Disaggregation Of Revenue [Line Items]
Revenue									577.5	623.9
Permanent Recruitment | Americas
Disaggregation Of Revenue [Line Items]
Revenue									123.4	117.6
Permanent Recruitment | Americas | United States
Disaggregation Of Revenue [Line Items]
Revenue									98.0	92.9
Permanent Recruitment | Americas | Other Americas
Disaggregation Of Revenue [Line Items]
Revenue									25.4	24.7
Permanent Recruitment | Southern Europe
Disaggregation Of Revenue [Line Items]
Revenue									148.6	145.6
Permanent Recruitment | Southern Europe | France
Disaggregation Of Revenue [Line Items]
Revenue									52.1	54.9
Permanent Recruitment | Southern Europe | Italy
Disaggregation Of Revenue [Line Items]
Revenue									37.6	35.9
Permanent Recruitment | Southern Europe | Other Southern Europe
Disaggregation Of Revenue [Line Items]
Revenue									58.9	54.8
Permanent Recruitment | Northern Europe
Disaggregation Of Revenue [Line Items]
Revenue									145.7	164.4
Permanent Recruitment | APME
Disaggregation Of Revenue [Line Items]
Revenue									159.8	196.3
Permanent Recruitment | Right Management
Disaggregation Of Revenue [Line Items]
Revenue									0.0	0.0
Other
Disaggregation Of Revenue [Line Items]
Revenue									337.5	351.9
Other | Americas
Disaggregation Of Revenue [Line Items]
Revenue									95.6	95.4
Other | Americas | United States
Disaggregation Of Revenue [Line Items]
Revenue									91.3	91.4
Other | Americas | Other Americas
Disaggregation Of Revenue [Line Items]
Revenue									4.3	4.0
Other | Southern Europe
Disaggregation Of Revenue [Line Items]
Revenue									46.4	58.2
Other | Southern Europe | France
Disaggregation Of Revenue [Line Items]
Revenue									15.7	21.6
Other | Southern Europe | Italy
Disaggregation Of Revenue [Line Items]
Revenue									22.4	20.7
Other | Southern Europe | Other Southern Europe
Disaggregation Of Revenue [Line Items]
Revenue									8.3	15.9
Other | Northern Europe
Disaggregation Of Revenue [Line Items]
Revenue									32.4	33.6
Other | APME
Disaggregation Of Revenue [Line Items]
Revenue									13.0	15.1
Other | Right Management
Disaggregation Of Revenue [Line Items]
Revenue									$ 150.1	$ 149.6