Segment Data - Schedule of Segment Data (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenues from services	$ 4,584,800,000	$ 5,248,900,000	$ 12,946,100,000	$ 15,666,900,000
Operating unit profit	97,300,000	253,400,000	219,800,000	628,700,000
Corporate expenses	(29,100,000)	(29,200,000)	(83,500,000)	(89,000,000.0)
Goodwill impairment charges	0		(66,800,000)	(64,000,000.0)
Intangible asset amortization expense	(6,600,000)	(7,200,000)	(20,200,000)	(22,400,000)
Operating profit	61,600,000	217,000,000.0	49,300,000	453,300,000
Interest and other (expenses) income, net	(6,000,000.0)	(12,200,000)	(32,300,000)	46,100,000
Earnings before income taxes	55,600,000	204,800,000	17,000,000.0	499,400,000
Americas
Segment Reporting Information [Line Items]
Revenues from services	929,100,000	1,094,500,000	2,776,700,000	3,192,900,000
Operating unit profit	31,500,000	59,300,000	67,800,000	146,600,000
Southern Europe
Segment Reporting Information [Line Items]
Revenues from services	2,112,400,000	2,357,800,000	5,527,900,000	6,860,900,000
Operating unit profit	71,600,000	116,400,000	136,700,000	326,600,000
Northern Europe
Segment Reporting Information [Line Items]
Revenues from services	947,700,000	1,166,800,000	2,881,900,000	3,561,400,000
Operating unit profit	(22,500,000)	22,800,000	(36,300,000)	50,300,000
APME
Segment Reporting Information [Line Items]
Revenues from services	595,600,000	629,800,000	1,759,600,000	2,051,700,000
Operating unit profit	16,700,000	54,900,000	51,600,000	105,200,000
United States | Americas
Segment Reporting Information [Line Items]
Revenues from services	578,800,000	666,400,000	1,705,600,000	1,942,300,000
Other Americas | Americas
Segment Reporting Information [Line Items]
Revenues from services	350,300,000	428,100,000	1,071,100,000	1,250,600,000
France | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services	1,205,300,000	1,386,400,000	3,035,100,000	4,117,800,000
Italy | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services	351,200,000	376,900,000	947,400,000	1,127,700,000
Other Southern Europe | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services	555,900,000	594,500,000	1,545,400,000	1,615,400,000
Reportable subsegments | United States | Americas
Segment Reporting Information [Line Items]
Revenues from services	578,800,000	666,400,000	1,705,600,000	1,942,300,000
Operating unit profit	19,300,000	39,800,000	30,800,000	94,100,000
Reportable subsegments | Other Americas | Americas
Segment Reporting Information [Line Items]
Revenues from services	350,300,000	428,100,000	1,071,100,000	1,250,600,000
Operating unit profit	12,200,000	19,500,000	37,000,000.0	52,500,000
Reportable subsegments | France | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services	1,205,300,000	1,386,400,000	3,035,100,000	4,117,800,000
Operating unit profit	51,500,000	70,300,000	87,000,000.0	201,300,000
Reportable subsegments | Italy | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services	351,200,000	376,900,000	947,400,000	1,127,700,000
Operating unit profit	15,400,000	23,300,000	40,600,000	73,400,000
Reportable subsegments | Other Southern Europe | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services	555,900,000	594,500,000	1,545,400,000	1,615,400,000
Operating unit profit	$ 4,700,000	$ 22,800,000	$ 9,100,000	$ 51,900,000