Business Segments (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ 1,043.5	$ 559.1	$ 715.4
Restructuring and acquisition charges	(84.7)	(142.4)	(184.4)
Depreciation and amortization	217.5	226.4	202.4
Equity in earnings (losses)	(209.4)	(8.0)	(36.3)
Segment revenue:
Revenues	19,367.0	16,589.9	17,983.2
Revenue Before Reimbursements	11,045.6	8,900.1	10,030.6
Americas [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	138.5	151.4	126.5
Equity in earnings (losses)	(142.2)	(19.0)	(1.7)
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	1,073.8	554.9	613.1
Segment revenue:
Revenues	11,887.1	10,005.2	10,578.9
EMEA [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	42.2	40.0	43.1
Equity in earnings (losses)	0.0	0.0	1.0
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	(5.6)	(16.0)	46.0
Segment revenue:
Revenues	3,477.7	3,092.8	3,470.7
Asia Pacific [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	30.2	27.8	26.4
Equity in earnings (losses)	(4.6)	(1.4)	(2.3)
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	130.2	116.4	129.6
Segment revenue:
Revenues	3,502.1	3,070.1	3,407.5
LaSalle Investment Management [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	6.6	7.2	6.4
Equity in earnings (losses)	(62.6)	12.4	(33.3)
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	139.2	54.2	147.4
Segment revenue:
Revenues	500.1	421.8	526.1
Leasing [Member] \| Americas [Member]
Segment revenue:
Revenues	2,179.1	1,451.2	1,960.5
Leasing [Member] \| EMEA [Member]
Segment revenue:
Revenues	347.3	245.6	302.7
Leasing [Member] \| Asia Pacific [Member]
Segment revenue:
Revenues	267.6	187.4	260.8
Capital Markets [Member] \| Americas [Member]
Segment revenue:
Revenues	1,525.9	952.0	914.2
Capital Markets [Member] \| EMEA [Member]
Segment revenue:
Revenues	473.4	330.7	411.9
Capital Markets [Member] \| Asia Pacific [Member]
Segment revenue:
Revenues	194.2	124.7	216.1
Property & Facility Management [Member] \| Americas [Member]
Segment revenue:
Revenues	6,375.3	5,878.2	5,607.9
Property & Facility Management [Member] \| EMEA [Member]
Segment revenue:
Revenues	1,504.7	1,456.6	1,551.6
Property & Facility Management [Member] \| Asia Pacific [Member]
Segment revenue:
Revenues	2,342.7	2,150.6	2,205.2
Project & Development Services [Member] \| Americas [Member]
Segment revenue:
Revenues	1,366.3	1,333.5	1,689.6
Project & Development Services [Member] \| EMEA [Member]
Segment revenue:
Revenues	870.8	785.6	904.3
Project & Development Services [Member] \| Asia Pacific [Member]
Segment revenue:
Revenues	447.8	411.0	527.6
Advisory, Consulting and Other [Member] \| Americas [Member]
Segment revenue:
Revenues	440.5	390.3	406.7
Advisory, Consulting and Other [Member] \| EMEA [Member]
Segment revenue:
Revenues	281.5	274.3	300.2
Advisory, Consulting and Other [Member] \| Asia Pacific [Member]
Segment revenue:
Revenues	249.8	196.4	197.8
Advisory Fees [Member] \| LaSalle Investment Management [Member]
Segment revenue:
Revenues	364.7	336.4	326.4
Transaction Fees & Other [Member] \| LaSalle Investment Management [Member]
Segment revenue:
Revenues	41.5	43.9	61.3
Incentive Fees [Member] \| LaSalle Investment Management [Member]
Segment revenue:
Revenues	$ 93.9	$ 41.5	$ 138.4

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