Business Segments (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ 292.9	$ 154.9	$ 597.9	$ 230.3
Restructuring and acquisition charges	(15.6)	(33.5)	(50.9)	(75.8)
Depreciation and amortization	52.8	54.9	160.3	166.8
Equity in earnings (losses)	(17.4)	(15.0)	(106.7)	(1.4)
Segment revenue:
Revenue Before Reimbursements	2,804.9	2,091.4	7,442.0	6,152.9
Revenue	4,889.2	3,978.1	13,421.3	11,744.5
Exclusive of LaSalle Investment Company II [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	1.8	1.8	5.1	5.4
Equity in earnings (losses)	(8.4)	(8.5)	(44.7)	20.5
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	29.7	29.5	88.9	28.8
Segment revenue:
Revenue	129.8	110.3	336.8	315.1
Americas [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	33.2	36.4	100.6	112.9
Equity in earnings (losses)	(7.6)	(4.8)	(58.6)	(20.4)
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	285.6	140.0	637.2	277.1
Segment revenue:
Revenue	3,083.2	2,380.8	8,228.6	7,132.9
EMEA [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	10.3	9.8	32.2	28.0
Equity in earnings (losses)	0.0	(0.8)	0.0	(0.8)
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	(10.3)	(1.9)	(45.8)	(55.8)
Segment revenue:
Revenue	830.0	743.8	2,356.0	2,125.9
Asia Pacific [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	7.5	6.9	22.4	20.5
Equity in earnings (losses)	(1.4)	(0.9)	(3.4)	(0.7)
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	20.9	35.8	75.2	57.4
Segment revenue:
Revenue	846.2	743.2	2,499.9	2,170.6
Leasing [Member] \| Americas [Member]
Segment revenue:
Revenue	594.6	325.2	1,442.5	1,022.2
Leasing [Member] \| EMEA [Member]
Segment revenue:
Revenue	80.6	58.5	209.0	153.3
Leasing [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	63.6	46.6	147.7	105.8
Capital Markets [Member] \| Americas [Member]
Segment revenue:
Revenue	421.2	190.8	934.1	579.0
Capital Markets [Member] \| EMEA [Member]
Segment revenue:
Revenue	110.4	73.3	284.0	196.5
Capital Markets [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	43.4	20.4	125.7	66.1
Property & Facility Management [Member] \| Americas [Member]
Segment revenue:
Revenue	1,564.8	1,433.3	4,576.8	4,331.6
Property & Facility Management [Member] \| EMEA [Member]
Segment revenue:
Revenue	368.5	348.7	1,074.5	1,044.2
Property & Facility Management [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	567.3	521.3	1,729.9	1,568.5
Project & Development Services [Member] \| Americas [Member]
Segment revenue:
Revenue	389.2	336.2	962.0	917.9
Project & Development Services [Member] \| EMEA [Member]
Segment revenue:
Revenue	207.2	190.8	595.7	558.3
Project & Development Services [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	111.5	104.5	319.7	296.5
Advisory, Consulting and Other [Member] \| Americas [Member]
Segment revenue:
Revenue	113.4	95.3	313.2	282.2
Advisory, Consulting and Other [Member] \| EMEA [Member]
Segment revenue:
Revenue	63.3	72.5	192.8	173.6
Advisory, Consulting and Other [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	60.4	50.4	176.9	133.7
Advisory Fees [Member] \| Exclusive of LaSalle Investment Company II [Member]
Segment revenue:
Revenue	97.3	84.6	270.4	251.1
Transaction Fees & Other [Member] \| Exclusive of LaSalle Investment Company II [Member]
Segment revenue:
Revenue	10.2	17.7	28.9	36.1
Incentive Fees [Member] \| Exclusive of LaSalle Investment Company II [Member]
Segment revenue:
Revenue	$ 22.3	$ 8.0	$ 37.5	$ 27.9

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