Supplementary Data (Details) - USD ($)		3 Months Ended		9 Months Ended
	Jan. 01, 2024	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Text Block [Abstract]
Goodwill	$ 5,080,900,000	$ 4,738,700,000		$ 4,738,700,000		$ 5,080,900,000
Goodwill, Acquired During Period				(117,600,000)
Goodwill, Other Decrease				7,500,000
Goodwill	5,080,900,000	4,738,700,000		4,738,700,000		5,080,900,000
Goodwill, Transfers	0
Goodwill, Acquired During Period				(117,600,000)
Goodwill, Impairment Loss		(232,100,000)	$ 0	(232,100,000)	$ 0
Goodwill, Other Decrease				7,500,000
Restructuring charges		500,000	(600,000)	1,400,000	(700,000)
Operating lease right-of-use asset		1,062,100,000		1,062,100,000		1,162,600,000
Disposal Group, Including Discontinued Operation, Assets, Net of Valuation Allowance		223,400,000		223,400,000
Disposal Group, Including Discontinued Operation, Accounts Payable		18,500,000		18,500,000
Disposal Group, Including Discontinued Operation, Accrued Liabilities		6,100,000		6,100,000
Disposal Group, Including Discontinued Operation, Current porting of oprerating leases		3,100,000		3,100,000
Disposal Group, Including Discontinued Operation, Non-current operating leases		31,700,000		31,700,000
Disposal Group, Including Discontinued Operation, Other Liabilities, Noncurrent		700,000		700,000
Disposal Group, Including Discontinued Operation, Liabilities		60,100,000		60,100,000
Disposal Group, Including Discontinued Operation, Net Assets		163,300,000		163,300,000
Goodwill, Impaired, Accumulated Impairment Loss		207,200,000		207,200,000
Valuation Allowance for Impairment of Recognized Servicing Assets, Sales and Disposals				(15,400,000)
Disposal Group, Including Discontinued Operation, Cash and Cash Equivalents		7,400,000		7,400,000
Accounts Receivable, Held-for-Sale		36,200,000		36,200,000
Accounts receivable, billable to clients, Held-for-Sale		27,100,000		27,100,000
Disposal Group, Including Discontinued Operation, Goodwill		116,600,000		116,600,000
Disposal Group, Including Discontinued Operation, Property, Plant and Equipment		15,400,000		15,400,000
Disposal Group, Including Discontinued Operation, Operating lease right-of-use assets		30,500,000		30,500,000
Disposal Group, Including Discontinued Operation, Other Assets		5,600,000		5,600,000
Disposal Group, Including Discontinued Operation, Assets		238,800,000		$ 238,800,000
Minimum [Member]
Goodwill testing discount rate				14.50%
Measurement Input, Revenue Growth Rate 1-5yrs				(2.00%)
Measurement Input, Terminal Revenue Growth Rate				2.50%
Maximum [Member]
Goodwill testing discount rate				22.00%
Measurement Input, Revenue Growth Rate 1-5yrs				6.00%
Measurement Input, Terminal Revenue Growth Rate				3.00%
MD&E
Text Block [Abstract]
Goodwill	2,690,800,000	2,347,200,000		$ 2,347,200,000		2,677,500,000
Goodwill, Acquired During Period				(116,500,000)
Goodwill, Other Decrease				5,000,000.0
Goodwill	2,690,800,000	2,347,200,000		2,347,200,000		2,677,500,000
Goodwill, Transfers	13,300,000
Goodwill, Acquired During Period				(116,500,000)
Goodwill, Impairment Loss		(232,100,000)	0	(232,100,000)	0
Goodwill, Other Decrease				5,000,000.0
Lease impairment costs		400,000	(100,000)	600,000	(1,300,000)
Restructuring charges		500,000	(100,000)	800,000	(1,300,000)
Disposal Group, Including Discontinued Operation, Goodwill		24,900,000		24,900,000
IA&C
Text Block [Abstract]
Goodwill	1,709,900,000	1,709,500,000		1,709,500,000		1,723,200,000
Goodwill, Acquired During Period				(1,200,000)
Goodwill, Other Decrease				800,000
Goodwill	1,709,900,000	1,709,500,000		1,709,500,000		1,723,200,000
Goodwill, Transfers	(13,300,000)
Goodwill, Acquired During Period				(1,200,000)
Goodwill, Impairment Loss				0
Goodwill, Other Decrease				800,000
Lease impairment costs			(600,000)	300,000	(800,000)
Restructuring charges		0	(500,000)	300,000	(200,000)
SC&E
Text Block [Abstract]
Goodwill	680,200,000	682,000,000.0		682,000,000.0		680,200,000
Goodwill, Acquired During Period				100,000
Goodwill, Other Decrease				1,700,000
Goodwill	680,200,000	682,000,000.0		682,000,000.0		$ 680,200,000
Goodwill, Transfers	$ 0
Goodwill, Acquired During Period				100,000
Goodwill, Impairment Loss				0
Goodwill, Other Decrease				1,700,000
Lease impairment costs			(200,000)	300,000
Restructuring charges		$ 0	$ (200,000)	$ 300,000	$ 700,000