Business Segments (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ 224.3	$ 10.8	$ 305.0	$ 75.4
Restructuring and acquisition charges	(18.1)	(28.2)	(35.3)	(42.3)
Depreciation and amortization	54.5	56.9	107.5	111.9
Equity in earnings (losses)	(40.8)	(14.7)	(89.3)	13.6
Segment revenue:
Revenue Before Reimbursements	2,507.5	1,828.5	4,637.1	4,061.5
Revenue	4,495.0	3,670.4	8,532.1	7,766.4
Exclusive of LaSalle Investment Company II [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	1.6	1.8	3.3	3.6
Equity in earnings (losses)	(23.3)	(11.3)	(36.3)	29.0
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	39.8	25.4	59.2	(0.7)
Segment revenue:
Revenue	115.8	99.9	207.0	204.8
Americas [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	34.3	39.1	67.4	76.5
Equity in earnings (losses)	(16.5)	(2.9)	(51.0)	(15.6)
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	206.6	42.5	351.6	137.1
Segment revenue:
Revenue	2,701.5	2,229.0	5,145.4	4,752.1
EMEA [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	11.0	9.0	21.9	18.2
Equity in earnings (losses)	0.0	0.0	0.0	0.0
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	0.3	(33.4)	(35.5)	(53.9)
Segment revenue:
Revenue	809.8	626.2	1,526.0	1,382.1
Asia Pacific [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	7.6	7.0	14.9	13.6
Equity in earnings (losses)	(1.0)	(0.5)	(2.0)	0.2
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	36.5	19.2	54.3	21.6
Segment revenue:
Revenue	867.9	715.3	1,653.7	1,427.4
Leasing [Member] \| Americas [Member]
Segment revenue:
Revenue	484.2	278.1	847.9	697.0
Leasing [Member] \| EMEA [Member]
Segment revenue:
Revenue	74.0	46.7	128.4	94.8
Leasing [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	52.7	33.8	84.1	59.2
Capital Markets [Member] \| Americas [Member]
Segment revenue:
Revenue	296.0	140.8	512.9	388.2
Capital Markets [Member] \| EMEA [Member]
Segment revenue:
Revenue	99.4	50.0	173.6	123.2
Capital Markets [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	53.0	24.0	82.3	45.7
Property & Facility Management [Member] \| Americas [Member]
Segment revenue:
Revenue	1,517.2	1,439.5	3,012.0	2,898.3
Property & Facility Management [Member] \| EMEA [Member]
Segment revenue:
Revenue	361.9	320.5	706.0	695.5
Property & Facility Management [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	587.5	515.2	1,162.6	1,047.2
Project & Development Services [Member] \| Americas [Member]
Segment revenue:
Revenue	303.0	275.3	572.8	581.7
Project & Development Services [Member] \| EMEA [Member]
Segment revenue:
Revenue	204.8	164.3	388.5	367.5
Project & Development Services [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	109.9	97.2	208.2	192.0
Advisory, Consulting and Other [Member] \| Americas [Member]
Segment revenue:
Revenue	101.1	95.3	199.8	186.9
Advisory, Consulting and Other [Member] \| EMEA [Member]
Segment revenue:
Revenue	69.7	44.7	129.5	101.1
Advisory, Consulting and Other [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	64.8	45.1	116.5	83.3
Advisory Fees [Member] \| Exclusive of LaSalle Investment Company II [Member]
Segment revenue:
Revenue	89.6	80.9	173.1	166.5
Transaction Fees & Other [Member] \| Exclusive of LaSalle Investment Company II [Member]
Segment revenue:
Revenue	11.0	4.8	18.7	18.4
Incentive Fees [Member] \| Exclusive of LaSalle Investment Company II [Member]
Segment revenue:
Revenue	$ 15.2	$ 14.2	$ 15.2	$ 19.9

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