Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 5,418.7	$ 16,598.0
Americas
Disaggregation of Revenue [Line Items]
Revenues	1,040.0	3,115.1
Southern Europe
Disaggregation of Revenue [Line Items]
Revenues	2,331.6	7,077.6
Northern Europe
Disaggregation of Revenue [Line Items]
Revenues	1,287.1	4,097.9
APME
Disaggregation of Revenue [Line Items]
Revenues	713.0	2,158.0
Right Management
Disaggregation of Revenue [Line Items]
Revenues	47.0	149.4
Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenues	4,812.1	14,727.6
Staffing and Interim | Americas
Disaggregation of Revenue [Line Items]
Revenues	944.2	2,827.0
Staffing and Interim | Southern Europe
Disaggregation of Revenue [Line Items]
Revenues	2,146.7	6,491.5
Staffing and Interim | Northern Europe
Disaggregation of Revenue [Line Items]
Revenues	1,138.8	3,628.0
Staffing and Interim | APME
Disaggregation of Revenue [Line Items]
Revenues	582.4	1,781.1
Staffing and Interim | Right Management
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenues	372.4	1,130.0
Outcome-Based Solutions and Consulting | Americas
Disaggregation of Revenue [Line Items]
Revenues	42.9	130.3
Outcome-Based Solutions and Consulting | Southern Europe
Disaggregation of Revenue [Line Items]
Revenues	137.6	430.5
Outcome-Based Solutions and Consulting | Northern Europe
Disaggregation of Revenue [Line Items]
Revenues	104.3	320.2
Outcome-Based Solutions and Consulting | APME
Disaggregation of Revenue [Line Items]
Revenues	75.4	212.8
Outcome-Based Solutions and Consulting | Right Management
Disaggregation of Revenue [Line Items]
Revenues	12.2	36.2
Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenues	152.4	479.0
Permanent Recruitment | Americas
Disaggregation of Revenue [Line Items]
Revenues	29.8	88.5
Permanent Recruitment | Southern Europe
Disaggregation of Revenue [Line Items]
Revenues	34.2	111.6
Permanent Recruitment | Northern Europe
Disaggregation of Revenue [Line Items]
Revenues	36.6	126.1
Permanent Recruitment | APME
Disaggregation of Revenue [Line Items]
Revenues	51.8	152.8
Permanent Recruitment | Right Management
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Products and Services, Other
Disaggregation of Revenue [Line Items]
Revenues	81.8	261.4
Products and Services, Other | Americas
Disaggregation of Revenue [Line Items]
Revenues	23.1	69.3
Products and Services, Other | Southern Europe
Disaggregation of Revenue [Line Items]
Revenues	13.1	44.0
Products and Services, Other | Northern Europe
Disaggregation of Revenue [Line Items]
Revenues	7.4	23.6
Products and Services, Other | APME
Disaggregation of Revenue [Line Items]
Revenues	3.4	11.3
Products and Services, Other | Right Management
Disaggregation of Revenue [Line Items]
Revenues	34.8	113.2
Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	5,304.5	16,235.3
Transferred over time | Americas
Disaggregation of Revenue [Line Items]
Revenues	1,022.6	3,063.5
Transferred over time | Southern Europe
Disaggregation of Revenue [Line Items]
Revenues	2,301.0	6,976.1
Transferred over time | Northern Europe
Disaggregation of Revenue [Line Items]
Revenues	1,255.9	3,989.9
Transferred over time | APME
Disaggregation of Revenue [Line Items]
Revenues	678.0	2,056.4
Transferred over time | Right Management
Disaggregation of Revenue [Line Items]
Revenues	47.0	149.4
Transferred at point in time
Disaggregation of Revenue [Line Items]
Revenues	114.2	362.7
Transferred at point in time | Americas
Disaggregation of Revenue [Line Items]
Revenues	17.4	51.6
Transferred at point in time | Southern Europe
Disaggregation of Revenue [Line Items]
Revenues	30.6	101.5
Transferred at point in time | Northern Europe
Disaggregation of Revenue [Line Items]
Revenues	31.2	108.0
Transferred at point in time | APME
Disaggregation of Revenue [Line Items]
Revenues	35.0	101.6
Transferred at point in time | Right Management
Disaggregation of Revenue [Line Items]
Revenues	$ 0.0	$ 0.0