Revenue Recognition - Disaggregation of Revenue (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 5,522.4
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	5,401.0
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	121.4
Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	4,896.1
Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	378.5
Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	159.9
Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	87.9
Americas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,022.6
Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,006.4
Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	16.2
Americas | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	931.4
Americas | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	40.5
Americas | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	27.8
Americas | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	22.9
Southern Europe
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,312.0
Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,276.5
Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	35.5
Southern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,110.3
Southern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	148.3
Southern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	38.6
Southern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	14.8
Northern Europe
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,417.6
Northern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,379.2
Northern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	38.4
Northern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,255.2
Northern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	109.8
Northern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	44.5
Northern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	8.1
APME
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	720.2
APME | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	688.9
APME | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	31.3
APME | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	599.2
APME | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	68.4
APME | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	49.0
APME | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	3.6
Right Management
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	50.0
Right Management | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	50.0
Right Management | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0
Right Management | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0
Right Management | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	11.5
Right Management | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0
Right Management | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 38.5