Segment Data - Schedule of Segment Data (Details) - USD ($)	3 Months Ended			12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues from services				$ 20,724,400,000	$ 18,001,000,000.0	$ 20,863,500,000
Corporate expenses				(154,300,000)	(113,900,000)	(121,900,000)
Goodwill Impairment charge	$ 0	$ 0	$ 0	0	(66,800,000)	(64,000,000.0)
Intangible asset amortization expense				(24,200,000)	(27,200,000)	(29,800,000)
Operating profit				585,400,000	187,600,000	644,900,000
Interest and other (expenses) income, net				(17,300,000)	(39,900,000)	40,600,000
Earnings before income taxes				568,100,000	147,700,000	685,500,000
Depreciation and Amortization Expense				73,400,000	76,300,000	77,200,000
Earnings from Equity Investments				11,800,000	4,600,000	50,500,000
Total Assets				9,828,900,000	9,328,200,000	9,223,800,000
Equity Investments				114,200,000	106,600,000	97,800,000
Additions to Long-Lived Assets				528,400,000	555,000,000.0	599,400,000
Corporate
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense				11,800,000	4,700,000	3,100,000
Earnings from Equity Investments				11,800,000	4,700,000	(3,100,000)
Total Assets				396,600,000	503,200,000	489,400,000
Equity Investments				114,000,000.0	106,500,000
Long-Lived Assets				14,000,000.0	6,500,000	200,000
Segment Reconciling Items
Segment Reporting Information [Line Items]
Intangible asset amortization expense				(24,200,000)	(27,200,000)	(29,800,000)
Intercompany Eliminations
Segment Reporting Information [Line Items]
Revenues from services				(37,700,000)	0	0
Americas
Segment Reporting Information [Line Items]
Revenues from services				4,263,700,000	3,792,400,000	4,278,600,000
Operating Unit Profit (Loss)				195,200,000	116,000,000.0	203,400,000
Earnings from Equity Investments				0	0	0
Americas | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense				9,300,000	10,100,000	11,000,000.0
Total Assets				3,831,600,000	2,485,100,000	2,536,200,000
Equity Investments				0		0	$ 0
Americas | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to Long-Lived Assets				93,200,000	93,500,000	121,300,000
Southern Europe
Segment Reporting Information [Line Items]
Revenues from services				9,346,800,000	7,855,200,000	9,194,400,000
Operating Unit Profit (Loss)				416,300,000	237,000,000.0	455,300,000
Earnings from Equity Investments				(0)	100,000	47,400,000
Southern Europe | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense				20,000,000.0	21,800,000	20,600,000
Total Assets				3,691,200,000	4,167,200,000	3,975,500,000
Equity Investments				100,000	100,000	300,000
Southern Europe | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to Long-Lived Assets				237,300,000	251,800,000	273,800,000
Northern Europe
Segment Reporting Information [Line Items]
Revenues from services				4,670,500,000	3,976,700,000	4,735,500,000
Operating Unit Profit (Loss)				(67,800,000)	(27,600,000)	74,400,000
Earnings from Equity Investments				0	0	0
Northern Europe | Reportable segments
Segment Reporting Information [Line Items]
Goodwill Impairment charge					(66,800,000)
Depreciation and Amortization Expense				10,900,000	8,900,000	9,800,000
Total Assets				1,095,700,000	1,366,700,000	1,478,500,000
Equity Investments				0	0	0
Northern Europe | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to Long-Lived Assets				106,600,000	127,100,000	141,700,000
APME
Segment Reporting Information [Line Items]
Revenues from services				2,481,100,000	2,376,700,000	2,655,000,000.0
Operating Unit Profit (Loss)				84,600,000	70,100,000	127,500,000
Earnings from Equity Investments				0	0	0
APME | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense				8,300,000	8,200,000	5,900,000
Total Assets				813,800,000	806,000,000.0	744,200,000
Equity Investments				100,000	0	0
Long-Lived Assets				77,300,000	76,100,000	62,400,000
Northern Europe And APME
Segment Reporting Information [Line Items]
Revenues from services				20,762,100,000	18,001,000,000.0	20,863,500,000
Operating Unit Profit (Loss)				763,900,000	395,500,000	860,600,000
United States
Segment Reporting Information [Line Items]
Revenues from services					2,327,200,000
United States | Americas
Segment Reporting Information [Line Items]
Revenues from services				2,743,300,000	2,327,200,000
Earnings from Equity Investments				0	0	0
United States | Americas | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense				6,900,000	7,800,000	8,600,000
Total Assets				3,434,600,000	2,103,800,000	2,153,800,000
Equity Investments				0		0	0
United States | Americas | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to Long-Lived Assets				76,900,000	73,900,000	100,800,000
Other Americas | Americas
Segment Reporting Information [Line Items]
Revenues from services				1,520,400,000	1,465,200,000
Earnings from Equity Investments				0	0	0
Other Americas | Americas | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense				2,400,000	2,300,000	2,400,000
Total Assets				397,000,000.0	381,300,000	382,400,000
Equity Investments				0		0	$ 0
Other Americas | Americas | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to Long-Lived Assets				16,300,000	19,600,000	20,500,000
France | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services				5,171,300,000	4,338,100,000
Earnings from Equity Investments				0	0	0
France | Southern Europe | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense				12,500,000	14,400,000	13,900,000
Total Assets				2,353,300,000	2,778,300,000	2,732,600,000
Equity Investments				0	0	0
Long-Lived Assets				135,000,000.0	147,800,000	157,400,000
Italy | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services				1,795,400,000	1,370,700,000
Earnings from Equity Investments				(0)	(100,000)	(100,000)
Italy | Southern Europe | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense				2,500,000	2,100,000	1,800,000
Total Assets				509,700,000	540,400,000	464,200,000
Equity Investments				100,000	100,000	300,000
Long-Lived Assets				33,600,000	36,000,000.0	35,100,000
Other Southern Europe | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services				2,380,100,000	2,146,400,000
Earnings from Equity Investments				0	0	47,500,000
Other Southern Europe | Southern Europe | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense				5,000,000.0	5,300,000	4,900,000
Total Assets				828,200,000	848,500,000	778,700,000
Equity Investments				0	0	0
Other Southern Europe | Southern Europe | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to Long-Lived Assets				68,700,000	68,000,000.0	81,300,000
Reportable subsegments | United States | Americas
Segment Reporting Information [Line Items]
Revenues from services				2,743,300,000	2,327,200,000	2,590,600,000
Operating Unit Profit (Loss)				136,000,000.0	60,900,000	128,000,000.0
Reportable subsegments | Other Americas | Americas
Segment Reporting Information [Line Items]
Revenues from services				1,520,400,000	1,465,200,000	1,688,000,000.0
Operating Unit Profit (Loss)				59,200,000	55,100,000	75,400,000
Reportable subsegments | France | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services				5,171,300,000	4,338,100,000	5,479,600,000
Operating Unit Profit (Loss)				233,500,000	149,000,000.0	284,900,000
Reportable subsegments | Italy | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services				1,795,400,000	1,370,700,000	1,508,300,000
Operating Unit Profit (Loss)				115,300,000	64,200,000	102,500,000
Reportable subsegments | Other Southern Europe | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services				2,380,100,000	2,146,400,000	2,206,500,000
Operating Unit Profit (Loss)				$ 67,500,000	$ 23,800,000	$ 67,900,000