Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenues from services	$ 5,393.2	$ 5,418.7	$ 5,656.9	$ 5,522.4	$ 5,637.5	$ 5,464.8	$ 5,174.8	$ 4,757.2	$ 21,991.2	$ 21,034.3	$ 19,654.1
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues from services									21,517.5
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from services									473.7
Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenues from services									19,498.2
Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenues from services									1,517.2
Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenues from services									623.9
Other
Disaggregation of Revenue [Line Items]
Revenues from services									351.9
Americas
Disaggregation of Revenue [Line Items]
Revenues from services									4,159.3	4,216.4	4,297.2
Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues from services									4,090.7
Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from services									68.6
Americas | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenues from services									3,769.4
Americas | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenues from services									176.9
Americas | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenues from services									117.6
Americas | Other
Disaggregation of Revenue [Line Items]
Revenues from services									95.4
Americas | United States
Disaggregation of Revenue [Line Items]
Revenues from services									2,522.3	2,659.0	2,836.8
Americas | United States | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues from services									2,471.0
Americas | United States | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from services									51.3
Americas | United States | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenues from services									2,208.3
Americas | United States | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenues from services									129.7
Americas | United States | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenues from services									92.9
Americas | United States | Other
Disaggregation of Revenue [Line Items]
Revenues from services									91.4
Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenues from services									1,637.0	1,557.4	1,460.4
Americas | Other Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues from services									1,619.7
Americas | Other Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from services									17.3
Americas | Other Americas | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenues from services									1,561.1
Americas | Other Americas | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenues from services									47.2
Americas | Other Americas | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenues from services									24.7
Americas | Other Americas | Other
Disaggregation of Revenue [Line Items]
Revenues from services									4.0
Southern Europe
Disaggregation of Revenue [Line Items]
Revenues from services									9,371.6	8,657.0	7,497.6
Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues from services									9,239.9
Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from services									131.7
Southern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenues from services									8,588.5
Southern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenues from services									579.3
Southern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenues from services									145.6
Southern Europe | Other
Disaggregation of Revenue [Line Items]
Revenues from services									58.2
Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenues from services									5,827.7	5,477.2	4,837.4
Southern Europe | France | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues from services									5,775.1
Southern Europe | France | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from services									52.6
Southern Europe | France | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenues from services									5,526.5
Southern Europe | France | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenues from services									224.7
Southern Europe | France | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenues from services									54.9
Southern Europe | France | Other
Disaggregation of Revenue [Line Items]
Revenues from services									21.6
Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenues from services									1,670.6	1,475.9	1,167.7
Southern Europe | Italy | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues from services									1,637.1
Southern Europe | Italy | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from services									33.5
Southern Europe | Italy | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenues from services									1,565.6
Southern Europe | Italy | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenues from services									48.4
Southern Europe | Italy | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenues from services									35.9
Southern Europe | Italy | Other
Disaggregation of Revenue [Line Items]
Revenues from services									20.7
Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenues from services									1,873.3	1,703.9	1,492.5
Southern Europe | Other Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues from services									1,827.7
Southern Europe | Other Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from services									45.6
Southern Europe | Other Southern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenues from services									1,496.4
Southern Europe | Other Southern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenues from services									306.2
Southern Europe | Other Southern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenues from services									54.8
Southern Europe | Other Southern Europe | Other
Disaggregation of Revenue [Line Items]
Revenues from services									15.9
Northern Europe
Disaggregation of Revenue [Line Items]
Revenues from services									5,370.5	5,306.4	5,129.1
Northern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues from services									5,229.1
Northern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from services									141.4
Northern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenues from services									4,746.3
Northern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenues from services									426.2
Northern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenues from services									164.4
Northern Europe | Other
Disaggregation of Revenue [Line Items]
Revenues from services									33.6
APME
Disaggregation of Revenue [Line Items]
Revenues from services									2,890.3	2,636.4	2,471.3
APME | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues from services									2,758.3
APME | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from services									132.0
APME | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenues from services									2,394.0
APME | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenues from services									284.9
APME | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenues from services									196.3
APME | Other
Disaggregation of Revenue [Line Items]
Revenues from services									15.1
Right Management
Disaggregation of Revenue [Line Items]
Revenues from services									199.5	$ 218.1	$ 258.9
Right Management | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues from services									199.5
Right Management | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from services									0.0
Right Management | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenues from services									0.0
Right Management | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenues from services									49.9
Right Management | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenues from services									0.0
Right Management | Other
Disaggregation of Revenue [Line Items]
Revenues from services									$ 149.6