Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 4,584.8	$ 5,248.9	$ 12,946.1	$ 15,666.9
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	4,513.2	5,148.5	12,719.9	15,332.7
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	71.6	100.4	226.2	334.2
Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	4,036.9	4,670.4	11,326.7	13,882.5
Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	359.5	360.2	1,059.9	1,088.3
Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	102.0	137.6	321.7	445.6
Other
Disaggregation of Revenue [Line Items]
Revenue	86.4	80.7	237.8	250.5
Americas
Disaggregation of Revenue [Line Items]
Revenue	929.1	1,094.5	2,776.7	3,192.9
Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	915.2	1,076.3	2,735.9	3,139.7
Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	13.9	18.2	40.8	53.2
Americas | United States
Disaggregation of Revenue [Line Items]
Revenue	578.8	666.4	1,705.6	1,942.3
Americas | United States | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	567.6	652.8	1,673.0	1,902.0
Americas | United States | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	11.2	13.6	32.6	40.3
Americas | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	350.3	428.1	1,071.1	1,250.6
Americas | Other Americas | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	347.6	423.5	1,062.9	1,237.7
Americas | Other Americas | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	2.7	4.6	8.2	12.9
Americas | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	810.2	969.1	2,444.6	2,827.3
Americas | Staffing and Interim | United States
Disaggregation of Revenue [Line Items]
Revenue	478.2	564.5	1,425.9	1,644.5
Americas | Staffing and Interim | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	332.0	404.6	1,018.7	1,182.8
Americas | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	43.9	52.1	124.2	151.2
Americas | Outcome-Based Solutions and Consulting | United States
Disaggregation of Revenue [Line Items]
Revenue	33.0	38.5	93.6	111.8
Americas | Outcome-Based Solutions and Consulting | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	10.9	13.6	30.6	39.4
Americas | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	25.3	31.9	74.7	90.9
Americas | Permanent Recruitment | United States
Disaggregation of Revenue [Line Items]
Revenue	21.3	25.2	62.1	72.2
Americas | Permanent Recruitment | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	4.0	6.7	12.6	18.7
Americas | Other
Disaggregation of Revenue [Line Items]
Revenue	49.7	41.4	133.2	123.5
Americas | Other | United States
Disaggregation of Revenue [Line Items]
Revenue	46.3	38.2	124.0	113.8
Americas | Other | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	3.4	3.2	9.2	9.7
Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	2,112.4	2,357.8	5,527.9	6,860.9
Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	2,090.6	2,326.4	5,457.8	6,758.8
Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	21.8	31.4	70.1	102.1
Southern Europe | France
Disaggregation of Revenue [Line Items]
Revenue	1,205.3	1,386.4	3,035.1	4,117.8
Southern Europe | France | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	1,196.4	1,375.1	3,008.0	4,080.1
Southern Europe | France | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	8.9	11.3	27.1	37.7
Southern Europe | Italy
Disaggregation of Revenue [Line Items]
Revenue	351.2	376.9	947.4	1,127.7
Southern Europe | Italy | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	345.6	369.6	929.7	1,101.2
Southern Europe | Italy | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	5.6	7.3	17.7	26.5
Southern Europe | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	555.9	594.5	1,545.4	1,615.4
Southern Europe | Other Southern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	548.6	581.7	1,520.1	1,577.5
Southern Europe | Other Southern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	7.3	12.8	25.3	37.9
Southern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	1,911.7	2,158.8	4,932.0	6,243.1
Southern Europe | Staffing and Interim | France
Disaggregation of Revenue [Line Items]
Revenue	1,119.8	1,309.8	2,781.5	3,880.0
Southern Europe | Staffing and Interim | Italy
Disaggregation of Revenue [Line Items]
Revenue	335.1	354.9	897.2	1,050.1
Southern Europe | Staffing and Interim | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	456.8	494.1	1,253.3	1,313.0
Southern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	166.4	150.0	486.3	453.1
Southern Europe | Outcome-Based Solutions and Consulting | France
Disaggregation of Revenue [Line Items]
Revenue	72.4	57.4	213.0	174.5
Southern Europe | Outcome-Based Solutions and Consulting | Italy
Disaggregation of Revenue [Line Items]
Revenue	7.0	10.4	20.7	32.1
Southern Europe | Outcome-Based Solutions and Consulting | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	87.0	82.2	252.6	246.5
Southern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	24.5	35.6	78.8	114.5
Southern Europe | Permanent Recruitment | France
Disaggregation of Revenue [Line Items]
Revenue	9.8	12.1	29.7	40.2
Southern Europe | Permanent Recruitment | Italy
Disaggregation of Revenue [Line Items]
Revenue	6.0	8.1	19.0	28.7
Southern Europe | Permanent Recruitment | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	8.7	15.4	30.1	45.6
Southern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue	9.8	13.4	30.8	50.2
Southern Europe | Other | France
Disaggregation of Revenue [Line Items]
Revenue	3.3	7.1	10.9	23.1
Southern Europe | Other | Italy
Disaggregation of Revenue [Line Items]
Revenue	3.1	3.5	10.5	16.8
Southern Europe | Other | Other Southern Europe
Disaggregation of Revenue [Line Items]
Revenue	3.4	2.8	9.4	10.3
Northern Europe
Disaggregation of Revenue [Line Items]
Revenue	947.7	1,166.8	2,881.9	3,561.4
Northern Europe | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	928.3	1,137.6	2,818.3	3,465.8
Northern Europe | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	19.4	29.2	63.6	95.6
Northern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	845.7	1,037.2	2,563.6	3,141.4
Northern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	63.2	79.6	198.5	259.4
Northern Europe | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	22.2	33.5	73.1	109.8
Northern Europe | Other
Disaggregation of Revenue [Line Items]
Revenue	16.6	16.5	46.7	50.8
APME
Disaggregation of Revenue [Line Items]
Revenue	595.6	629.8	1,759.6	2,051.7
APME | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue	579.1	608.2	1,707.9	1,968.4
APME | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue	16.5	21.6	51.7	83.3
APME | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue	469.3	505.3	1,386.5	1,670.7
APME | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue	86.0	78.5	250.9	224.6
APME | Permanent Recruitment
Disaggregation of Revenue [Line Items]
Revenue	30.0	36.6	95.1	130.4
APME | Other
Disaggregation of Revenue [Line Items]
Revenue	$ 10.3	$ 9.4	$ 27.1	$ 26.0