Restructuring and Acquisition Charges (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Restructuring Cost and Reserve [Line Items]
Restructuring and acquisition charges	$ 84.7	$ 142.4	$ 184.4
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	(2.6)	14.8	(28.4)
Share-based Payment Arrangement, Noncash Expense	17.8	36.7	28.0
Restructuring reserve [Roll Forward]
Restructuring reserve, beginning balance	45.0	36.5	15.1
Accruals	64.3	120.5	128.0
Payments made	(79.5)	(112.0)	(106.6)
Restructuring reserve, ending balance	29.8	45.0	36.5
Severance [Member]
Restructuring reserve [Roll Forward]
Restructuring reserve, beginning balance	41.3	24.3	14.0
Accruals	14.3	69.0	31.1
Payments made	(34.7)	(52.0)	(20.8)
Restructuring reserve, ending balance	20.9	41.3	24.3
Lease Exit [Member]
Restructuring reserve [Roll Forward]
Restructuring reserve, beginning balance	2.5	8.4	0.6
Accruals	11.6	25.0	11.1
Payments made	(13.2)	(30.9)	(3.3)
Restructuring reserve, ending balance	0.9	2.5	8.4
Other Restructuring and Acquisition Costs [Member]
Restructuring reserve [Roll Forward]
Restructuring reserve, beginning balance	1.2	3.8	0.5
Accruals	38.4	26.5	85.8
Payments made	(31.6)	(29.1)	(82.5)
Restructuring reserve, ending balance	8.0	1.2	3.8
Contract Termination and Other Charges
Restructuring reserve [Roll Forward]
Accruals	50.0	51.5	96.9
HFF Acquisition [Member]
Restructuring Cost and Reserve [Line Items]
Business Combination, Integration Related Costs	$ 40.0	$ 75.9	$ 115.1

Amount of increase (decrease) in the value of a contingent consideration liability, including, but not limited to, differences arising upon settlement.

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Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

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Amount, after cash payment, of expenses associated with exit or disposal activities pursuant to an authorized plan and expenses resulting from the write-down of assets. Excludes expenses related to a business combination, a discontinued operation or an asset retirement obligation.

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Amount of noncash expense for share-based payment arrangement.

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