Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 5,044.9	$ 5,522.4
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	4,931.5	5,401.0
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	113.4	121.4
Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	4,452.4	4,896.1
Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	358.8	378.5
Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	149.1	159.9
Other
Disaggregation of Revenue [Line Items]
Revenue	84.6	87.9
Americas
Disaggregation of Revenue [Line Items]
Revenue	1,007.3	1,022.6
Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	990.3	1,006.4
Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	17.0	16.2
Americas | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	914.1	931.4
Americas | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	42.7	40.5
Americas | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	28.7	27.8
Americas | Other
Disaggregation of Revenue [Line Items]
Revenue	21.8	22.9
Americas | United States
Disaggregation of Revenue [Line Items]
Revenue	603.6	616.3
Americas | United States | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	590.6	604.3
Americas | United States | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	13.0	12.0
Americas | United States | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	528.7	543.2
Americas | United States | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	31.0	29.3
Americas | United States | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	22.9	21.8
Americas | United States | Other
Disaggregation of Revenue [Line Items]
Revenue	21.0	22.0
Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	403.7	406.3
Americas | Other Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	399.7	402.1
Americas | Other Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	4.0	4.2
Americas | Other Americas | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	385.4	388.2
Americas | Other Americas | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	11.7	11.2
Americas | Other Americas | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	5.8	6.0
Americas | Other Americas | Other
Disaggregation of Revenue [Line Items]
Revenue	0.8	0.9
Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	2,102.2	2,312.0
Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	2,067.4	2,276.5
Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	34.8	35.5
Southern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	1,901.5	2,110.3
Southern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	146.9	148.3
Southern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	38.6	38.6
Southern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue	15.2	14.8
Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue	1,301.4	1,424.0
Southern Europe | France | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,287.7	1,409.6
Southern Europe | France | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	13.7	14.4
Southern Europe | France | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	1,224.5	1,344.6
Southern Europe | France | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	57.3	59.1
Southern Europe | France | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	14.5	14.9
Southern Europe | France | Other
Disaggregation of Revenue [Line Items]
Revenue	5.1	5.4
Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue	355.9	413.6
Southern Europe | Italy | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	346.6	404.8
Southern Europe | Italy | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	9.3	8.8
Southern Europe | Italy | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	330.0	387.0
Southern Europe | Italy | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	9.5	11.5
Southern Europe | Italy | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	9.9	9.2
Southern Europe | Italy | Other
Disaggregation of Revenue [Line Items]
Revenue	6.5	5.9
Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	444.9	474.4
Southern Europe | Other Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	433.1	462.1
Southern Europe | Other Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	11.8	12.3
Southern Europe | Other Southern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	347.0	378.7
Southern Europe | Other Southern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	80.1	77.7
Southern Europe | Other Southern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	14.2	14.5
Southern Europe | Other Southern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue	3.6	3.5
Northern Europe
Disaggregation of Revenue [Line Items]
Revenue	1,189.7	1,417.6
Northern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,156.6	1,379.2
Northern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	33.1	38.4
Northern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	1,053.4	1,255.2
Northern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	89.7	109.8
Northern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	38.1	44.5
Northern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue	8.5	8.1
APME
Disaggregation of Revenue [Line Items]
Revenue	699.9	720.2
APME | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	671.4	688.9
APME | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	28.5	31.3
APME | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	583.4	599.2
APME | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	69.4	68.4
APME | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	43.7	49.0
APME | Other
Disaggregation of Revenue [Line Items]
Revenue	3.4	3.6
Right Management
Disaggregation of Revenue [Line Items]
Revenue	45.8	50.0
Right Management | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	45.8	50.0
Right Management | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Right Management | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Right Management | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	10.1	11.5
Right Management | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Right Management | Other
Disaggregation of Revenue [Line Items]
Revenue	$ 35.7	$ 38.5