Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 5,277.1	$ 3,742.2	$ 10,201.5	$ 8,361.3
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	5,166.3	3,682.1	9,996.2	8,206.7
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	110.8	60.1	205.3	154.6
Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	4,634.0	3,254.1	8,944.9	7,289.8
Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	399.3	326.4	788.0	700.4
Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	155.7	86.4	291.4	219.7
Other
Disaggregation of Revenue [Line Items]
Revenue	88.1	75.3	177.2	151.4
Americas
Disaggregation of Revenue [Line Items]
Revenue	1,044.3	836.6	2,047.2	1,847.6
Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,022.1	826.4	2,007.0	1,820.7
Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	22.2	10.2	40.2	26.9
Americas | United States
Disaggregation of Revenue [Line Items]
Revenue	628.8	515.9	1,237.6	1,126.8
Americas | United States | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	611.7	507.6	1,206.4	1,105.4
Americas | United States | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	17.1	8.3	31.2	21.4
Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	415.5	320.7	809.6	720.8
Americas | Other Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	410.4	318.8	800.6	715.3
Americas | Other Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	5.1	1.9	9.0	5.5
Americas | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	920.8	736.2	1,805.8	1,634.4
Americas | Staffing and Interim | United States
Disaggregation of Revenue [Line Items]
Revenue	527.0	429.0	1,037.1	947.7
Americas | Staffing and Interim | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	393.8	307.2	768.7	686.7
Americas | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	44.5	37.8	83.2	80.3
Americas | Outcome-Based Solutions and Consulting | United States
Disaggregation of Revenue [Line Items]
Revenue	33.5	30.5	62.3	60.6
Americas | Outcome-Based Solutions and Consulting | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	11.0	7.3	20.9	19.7
Americas | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	39.8	18.9	74.4	49.4
Americas | Permanent Recruitment | United States
Disaggregation of Revenue [Line Items]
Revenue	31.9	15.8	60.4	40.8
Americas | Permanent Recruitment | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	7.9	3.1	14.0	8.6
Americas | Other
Disaggregation of Revenue [Line Items]
Revenue	39.2	43.7	83.8	83.5
Americas | Other | United States
Disaggregation of Revenue [Line Items]
Revenue	36.4	40.6	77.8	77.7
Americas | Other | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	2.8	3.1	6.0	5.8
Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	2,422.4	1,470.8	4,582.7	3,415.5
Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	2,386.8	1,453.6	4,516.1	3,367.2
Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	35.6	17.2	66.6	48.3
Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue	1,346.8	736.0	2,535.7	1,829.8
Southern Europe | France | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,333.1	729.5	2,510.0	1,811.6
Southern Europe | France | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	13.7	6.5	25.7	18.2
Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue	469.1	268.5	871.9	596.2
Southern Europe | Italy | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	458.1	264.1	851.6	584.1
Southern Europe | Italy | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	11.0	4.4	20.3	12.1
Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	606.5	466.3	1,175.1	989.5
Southern Europe | Other Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	595.6	460.0	1,154.5	971.5
Southern Europe | Other Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	10.9	6.3	20.6	18.0
Southern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	2,180.1	1,299.0	4,106.4	3,020.3
Southern Europe | Staffing and Interim | France
Disaggregation of Revenue [Line Items]
Revenue	1,241.1	663.4	2,325.1	1,661.7
Southern Europe | Staffing and Interim | Italy
Disaggregation of Revenue [Line Items]
Revenue	443.4	255.0	824.9	562.1
Southern Europe | Staffing and Interim | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	495.6	380.6	956.4	796.5
Southern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	186.1	142.5	372.9	319.9
Southern Europe | Outcome-Based Solutions and Consulting | France
Disaggregation of Revenue [Line Items]
Revenue	83.6	62.5	170.1	140.6
Southern Europe | Outcome-Based Solutions and Consulting | Italy
Disaggregation of Revenue [Line Items]
Revenue	8.2	5.0	15.9	13.7
Southern Europe | Outcome-Based Solutions and Consulting | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	94.3	75.0	186.9	165.6
Southern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	40.0	19.7	74.2	54.3
Southern Europe | Permanent Recruitment | France
Disaggregation of Revenue [Line Items]
Revenue	15.0	7.4	27.8	19.9
Southern Europe | Permanent Recruitment | Italy
Disaggregation of Revenue [Line Items]
Revenue	11.8	4.8	21.5	13.0
Southern Europe | Permanent Recruitment | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	13.2	7.5	24.9	21.4
Southern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue	16.2	9.6	29.2	21.0
Southern Europe | Other | France
Disaggregation of Revenue [Line Items]
Revenue	7.1	2.7	12.7	7.6
Southern Europe | Other | Italy
Disaggregation of Revenue [Line Items]
Revenue	5.7	3.7	9.6	7.4
Southern Europe | Other | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	3.4	3.2	6.9	6.0
Northern Europe
Disaggregation of Revenue [Line Items]
Revenue	1,190.5	865.7	2,324.3	1,934.2
Northern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,159.1	848.6	2,266.9	1,890.0
Northern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	31.4	17.1	57.4	44.2
Northern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	1,067.6	768.3	2,086.4	1,717.9
Northern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	66.6	64.4	131.4	135.3
Northern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	36.1	19.6	65.6	50.9
Northern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue	20.2	13.4	40.9	30.1
APME
Disaggregation of Revenue [Line Items]
Revenue	619.9	569.1	1,247.3	1,164.0
APME | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	598.3	553.5	1,206.2	1,128.8
APME | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	21.6	15.6	41.1	35.2
APME | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	465.5	450.6	946.3	917.2
APME | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	102.1	81.7	200.5	164.9
APME | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	39.8	28.2	77.2	65.1
APME | Other
Disaggregation of Revenue [Line Items]
Revenue	$ 12.5	$ 8.6	$ 23.3	$ 16.8