Segment Data - Schedule of Segment Data (Details) - USD ($)	3 Months Ended									12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues from Services	$ 5,054,900,000	$ 4,584,800,000	$ 3,742,200,000	$ 4,619,100,000	$ 5,196,600,000	$ 5,248,900,000	$ 5,373,100,000	$ 5,044,900,000		$ 18,001,000,000.0	$ 20,863,500,000	$ 21,991,200,000
Operating Unit Profit (Loss)										395,500,000	860,600,000	941,800,000
Corporate expenses										(113,900,000)	(121,900,000)	(110,000,000.0)
Impairment charge		$ 0				$ 0			$ 0	(66,800,000)	(64,000,000.0)
Intangible asset amortization expense										(27,200,000)	(29,800,000)	(35,100,000)
Interest and other (expenses) income, net										(39,900,000)	40,600,000	(42,000,000.0)
Earnings before income taxes										147,700,000	685,500,000	754,700,000
Depreciation and Amortization Expense										76,300,000	77,200,000	85,800,000
Earnings from Equity Investments										4,600,000	50,500,000	1,200,000
Total Assets	9,328,200,000				9,223,800,000					9,328,200,000	9,223,800,000	8,519,800,000
Equity Investments	106,600,000				97,800,000					106,600,000	97,800,000	161,400,000
Long-Lived Assets	154,300,000				150,900,000					154,300,000	150,900,000	160,000,000.0
Additions to Long-Lived Assets										47,200,000	48,700,000	60,300,000
Corporate
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense										100,000	100,000	100,000
Earnings from Equity Investments										4,700,000	3,100,000	(300,000)
Total Assets	503,200,000				489,400,000					503,200,000	489,400,000	520,900,000
Equity Investments	106,500,000				97,500,000					106,500,000	97,500,000	1,000,000.0
Long-Lived Assets	6,500,000				200,000					6,500,000	200,000	100,000
Additions to Long-Lived Assets										0	100,000	100,000
Segment Reconciling Items
Segment Reporting Information [Line Items]
Intangible asset amortization expense										27,200,000	29,800,000	35,100,000
Americas
Segment Reporting Information [Line Items]
Revenues from Services										3,792,400,000	4,278,600,000	4,258,900,000
Operating Unit Profit (Loss)										116,000,000.0	203,400,000	224,900,000
Earnings from Equity Investments										0	0	0
Americas | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense										10,100,000	11,000,000.0	12,300,000
Total Assets	2,485,100,000				2,536,200,000					2,485,100,000	2,536,200,000	2,232,100,000
Equity Investments	0				0					0	0	0
Long-Lived Assets	24,000,000.0				28,700,000					24,000,000.0	28,700,000	28,700,000
Americas | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to Long-Lived Assets										9,500,000	12,700,000	10,600,000
Southern Europe
Segment Reporting Information [Line Items]
Revenues from Services										7,855,200,000	9,194,400,000	9,399,900,000
Operating Unit Profit (Loss)										237,000,000.0	455,300,000	468,700,000
Earnings from Equity Investments										(100,000)	47,400,000	1,500,000
Southern Europe | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense										21,800,000	20,600,000	21,100,000
Total Assets	4,167,200,000				3,975,500,000					4,167,200,000	3,975,500,000	3,730,900,000
Equity Investments	100,000				300,000					100,000	300,000	158,100,000
Long-Lived Assets	71,800,000				75,600,000					71,800,000	75,600,000	78,500,000
Southern Europe | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to Long-Lived Assets										16,300,000	20,000,000.0	26,200,000
Northern Europe
Segment Reporting Information [Line Items]
Revenues from Services										3,976,700,000	4,735,500,000	5,417,500,000
Operating Unit Profit (Loss)										(27,600,000)	74,400,000	129,900,000
Earnings from Equity Investments										0	0	0
Northern Europe | Reportable segments
Segment Reporting Information [Line Items]
Impairment charge										(66,800,000)	(60,200,000)
Depreciation and Amortization Expense										8,900,000	9,800,000	11,400,000
Total Assets	1,366,700,000				1,478,500,000					1,366,700,000	1,478,500,000	1,240,900,000
Equity Investments	0				0					0	0	0
Long-Lived Assets	26,500,000				27,500,000					26,500,000	27,500,000	30,600,000
Northern Europe | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to Long-Lived Assets										7,300,000	7,700,000	16,600,000
APME
Segment Reporting Information [Line Items]
Revenues from Services										2,376,700,000	2,655,000,000.0	2,914,900,000
Operating Unit Profit (Loss)										70,100,000	127,500,000	118,300,000
Earnings from Equity Investments										0	0	0
APME | Reportable segments
Segment Reporting Information [Line Items]
Impairment charge											(3,800,000)
Depreciation and Amortization Expense										8,200,000	5,900,000	5,800,000
Total Assets	806,000,000.0				744,200,000					806,000,000.0	744,200,000	795,000,000.0
Equity Investments	0				0					0	0	2,300,000
Long-Lived Assets	25,500,000				18,900,000					25,500,000	18,900,000	22,100,000
APME | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to Long-Lived Assets										14,100,000	8,200,000	6,800,000
United States | Americas
Segment Reporting Information [Line Items]
Revenues from Services										2,327,200,000	2,590,600,000	2,608,900,000
Earnings from Equity Investments										0	0	0
United States | Americas | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense										7,800,000	8,600,000	9,800,000
Total Assets	2,103,800,000				2,153,800,000					2,103,800,000	2,153,800,000	1,888,100,000
Equity Investments	0				0					0	0	0
Long-Lived Assets	16,500,000				20,500,000					16,500,000	20,500,000	22,400,000
United States | Americas | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to Long-Lived Assets										7,000,000.0	8,300,000	7,700,000
Other Americas | Americas
Segment Reporting Information [Line Items]
Revenues from Services										1,465,200,000	1,688,000,000.0
Earnings from Equity Investments										0	0	0
Other Americas | Americas | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense										2,300,000	2,400,000	2,500,000
Total Assets	381,300,000				382,400,000					381,300,000	382,400,000	344,000,000.0
Equity Investments	0				0					0	0	0
Long-Lived Assets	7,500,000				8,200,000					7,500,000	8,200,000	6,300,000
Other Americas | Americas | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to Long-Lived Assets										2,500,000	4,400,000	2,900,000
France | Southern Europe
Segment Reporting Information [Line Items]
Revenues from Services										4,338,100,000	5,479,600,000
Earnings from Equity Investments										0	0	0
France | Southern Europe | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense										14,400,000	13,900,000	14,200,000
Total Assets	2,778,300,000				2,732,600,000					2,778,300,000	2,732,600,000	2,740,900,000
Equity Investments	0				0					0	0	0
Long-Lived Assets	44,800,000				46,900,000					44,800,000	46,900,000	50,100,000
France | Southern Europe | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to Long-Lived Assets										9,300,000	12,900,000	18,100,000
Italy | Southern Europe
Segment Reporting Information [Line Items]
Revenues from Services										1,370,700,000	1,508,300,000
Earnings from Equity Investments										(100,000)	(100,000)	(200,000)
Italy | Southern Europe | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense										2,100,000	1,800,000	1,900,000
Total Assets	540,400,000				464,200,000					540,400,000	464,200,000	406,500,000
Equity Investments	100,000				300,000					100,000	300,000	300,000
Long-Lived Assets	6,000,000.0				5,200,000					6,000,000.0	5,200,000	5,000,000.0
Italy | Southern Europe | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to Long-Lived Assets										2,600,000	2,100,000	2,200,000
Other Southern Europe | Southern Europe
Segment Reporting Information [Line Items]
Revenues from Services										2,146,400,000	2,206,500,000
Earnings from Equity Investments										0	47,500,000	1,700,000
Other Southern Europe | Southern Europe | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and Amortization Expense										5,300,000	4,900,000	5,000,000.0
Total Assets	848,500,000				778,700,000					848,500,000	778,700,000	583,500,000
Equity Investments	0				0					0	0	157,800,000
Long-Lived Assets	$ 21,000,000.0				$ 23,500,000					21,000,000.0	23,500,000	23,400,000
Other Southern Europe | Southern Europe | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to Long-Lived Assets										4,400,000	5,000,000.0	5,900,000
Reportable subsegments | United States | Americas
Segment Reporting Information [Line Items]
Operating Unit Profit (Loss)										60,900,000	128,000,000.0	149,900,000
Reportable subsegments | Other Americas | Americas
Segment Reporting Information [Line Items]
Revenues from Services										1,465,200,000	1,688,000,000.0	1,650,000,000.0
Operating Unit Profit (Loss)										55,100,000	75,400,000	75,000,000.0
Reportable subsegments | France | Southern Europe
Segment Reporting Information [Line Items]
Revenues from Services										4,338,100,000	5,479,600,000	5,846,400,000
Operating Unit Profit (Loss)										149,000,000.0	284,900,000	290,900,000
Reportable subsegments | Italy | Southern Europe
Segment Reporting Information [Line Items]
Revenues from Services										1,370,700,000	1,508,300,000	1,673,900,000
Operating Unit Profit (Loss)										64,200,000	102,500,000	111,300,000
Reportable subsegments | Other Southern Europe | Southern Europe
Segment Reporting Information [Line Items]
Revenues from Services										2,146,400,000	2,206,500,000	1,879,600,000
Operating Unit Profit (Loss)										$ 23,800,000	$ 67,900,000	$ 66,500,000