Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 4,619.1	$ 5,044.9
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	4,524.6	4,931.5
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	94.5	113.4
Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	4,035.7	4,452.4
Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	374.0	358.8
Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	133.3	149.1
Other
Disaggregation of Revenue [Line Items]
Revenue	76.1	84.6
Americas
Disaggregation of Revenue [Line Items]
Revenue	1,011.0	1,030.2
Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	994.3	1,013.2
Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	16.7	17.0
Americas | United States
Disaggregation of Revenue [Line Items]
Revenue	610.9	623.2
Americas | United States | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	597.8	610.2
Americas | United States | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	13.1	13.0
Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	400.1	407.0
Americas | Other Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	396.5	403.0
Americas | Other Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	3.6	4.0
Americas | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	898.2	914.1
Americas | Staffing and Interim | United States
Disaggregation of Revenue [Line Items]
Revenue	518.7	528.7
Americas | Staffing and Interim | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	379.5	385.4
Americas | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	42.5	47.4
Americas | Outcome-Based Solutions and Consulting | United States
Disaggregation of Revenue [Line Items]
Revenue	30.1	34.9
Americas | Outcome-Based Solutions and Consulting | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	12.4	12.5
Americas | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	30.5	28.7
Americas | Permanent Recruitment | United States
Disaggregation of Revenue [Line Items]
Revenue	25.0	22.9
Americas | Permanent Recruitment | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	5.5	5.8
Americas | Other
Disaggregation of Revenue [Line Items]
Revenue	39.8	40.0
Americas | Other | United States
Disaggregation of Revenue [Line Items]
Revenue	37.1	36.7
Americas | Other | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	2.7	3.3
Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	1,944.7	2,108.8
Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,913.6	2,073.9
Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	31.1	34.9
Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue	1,093.8	1,306.1
Southern Europe | France | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,082.1	1,292.4
Southern Europe | France | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	11.7	13.7
Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue	327.7	356.4
Southern Europe | Italy | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	320.0	347.1
Southern Europe | Italy | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	7.7	9.3
Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	523.2	446.3
Southern Europe | Other Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	511.5	434.4
Southern Europe | Other Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	11.7	11.9
Southern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	1,721.3	1,901.5
Southern Europe | Staffing and Interim | France
Disaggregation of Revenue [Line Items]
Revenue	998.3	1,224.5
Southern Europe | Staffing and Interim | Italy
Disaggregation of Revenue [Line Items]
Revenue	307.1	330.0
Southern Europe | Staffing and Interim | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	415.9	347.0
Southern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	177.4	149.1
Southern Europe | Outcome-Based Solutions and Consulting | France
Disaggregation of Revenue [Line Items]
Revenue	78.1	58.7
Southern Europe | Outcome-Based Solutions and Consulting | Italy
Disaggregation of Revenue [Line Items]
Revenue	8.7	9.8
Southern Europe | Outcome-Based Solutions and Consulting | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	90.6	80.6
Southern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	34.6	38.6
Southern Europe | Permanent Recruitment | France
Disaggregation of Revenue [Line Items]
Revenue	12.5	14.5
Southern Europe | Permanent Recruitment | Italy
Disaggregation of Revenue [Line Items]
Revenue	8.2	9.9
Southern Europe | Permanent Recruitment | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	13.9	14.2
Southern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue	11.4	19.6
Southern Europe | Other | France
Disaggregation of Revenue [Line Items]
Revenue	4.9	8.4
Southern Europe | Other | Italy
Disaggregation of Revenue [Line Items]
Revenue	3.7	6.7
Southern Europe | Other | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	2.8	4.5
Northern Europe
Disaggregation of Revenue [Line Items]
Revenue	1,068.5	1,200.5
Northern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,041.4	1,167.4
Northern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	27.1	33.1
Northern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	949.6	1,053.4
Northern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	70.9	91.5
Northern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	31.3	38.1
Northern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue	16.7	17.5
APME
Disaggregation of Revenue [Line Items]
Revenue	594.9	705.4
APME | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	575.3	677.0
APME | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	19.6	28.4
APME | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	466.6	583.4
APME | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	83.2	70.8
APME | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	36.9	43.7
APME | Other
Disaggregation of Revenue [Line Items]
Revenue	$ 8.2	$ 7.5