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Business Segments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
Business Segments (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting [Abstract]
Percentage of assets from countries having restrictions									5.00%	6.00%
Segment Reporting Information [Line Items]
Net investment activity (less than wholly-owned)									$ 79.2	$ 14.3	$ 2.3
Reimbursement Revenues									7,952.6	7,228.9	6,485.8
Segment revenue:
Revenues	$ 5,400.5	$ 4,495.6	$ 4,266.5	$ 3,820.6	$ 4,889.7	$ 3,969.8	$ 3,903.7	$ 3,555.2	17,983.2	16,318.4	14,453.2
Revenue Before Reimbursements									10,030.6	9,089.5	7,967.4
Segment Reporting Information Operating Expenses									9,130.8	8,343.8	7,390.8
Operating expenses:
Depreciation and amortization									202.4	186.1	167.2
Operating Income (Loss)	368.3	171.6	150.1	25.4	312.4	190.9	149.8	53.8	715.4	706.9	545.9
Equity in earnings (losses)									36.3	32.8	44.4
Segment Reconciling Items:
Segment Operating Expenses before Restructuring and acquisition									17,083.4	15,572.7	13,876.6
Segment Reporting Information Operating Income Before Restructuring Charges									899.8	745.7	576.6
Restructuring and acquisition charges	70.1	70.0	25.7	18.6	45.5	3.7	(11.1)	0.7	184.4	38.8	30.7
Operating Income (Loss)	368.3	$ 171.6	$ 150.1	$ 25.4	312.4	$ 190.9	$ 149.8	$ 53.8	715.4	706.9	545.9
Total Assets	13,672.6				10,025.5				13,672.6	10,025.5
Reportable Subsegments [Member]
Segment revenue:
Real Estate Revenues, Net									7,139.2	6,486.2	5,736.2
Operating expenses:
Gross contract vendor and subcontractor costs reimbursement									2,870.2	2,595.0	2,215.5
Reportable Subsegments [Member] \| Americas [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues									5,857.8	5,164.7	4,648.5
Segment revenue:
Revenues									10,578.9	9,041.0	8,015.1
Revenue Before Reimbursements									4,721.1	3,876.3	3,366.6
Derivative, Gain (Loss) on Derivative, Net									(21.2)	(8.3)	(15.7)
Real Estate Revenues, Net									3,899.7	3,215.1	2,829.9
Operating expenses:
Compensation, operating and administrative expenses									3,983.2	3,345.3	2,918.6
Depreciation and amortization									126.5	109.1	97.5
Operating Expenses, Excluding Reimbursed Expenses									4,109.7	3,454.4	3,016.1
Gross contract vendor and subcontractor costs reimbursement									800.2	652.9	521.0
Total fee based segment operating expenses									3,309.5	2,801.5	2,495.1
General Contractor Cost									(800.2)	(652.9)	(521.0)
Operating Income (Loss)									611.4	421.9	350.5
Equity in earnings (losses)									1.7	0.8	(0.2)
Operating Income (Loss), Including Income (Loss) from Equity Method Investments									613.1	422.7	350.3
Segment Reconciling Items:
Operating Income (Loss)									611.4	421.9	350.5
Total Assets	8,256.4				5,288.9				8,256.4	5,288.9
Reportable Subsegments [Member] \| EMEA [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues									709.7	677.5	471.9
Segment revenue:
Revenues									3,470.7	3,495.0	3,075.0
Revenue Before Reimbursements									2,761.0	2,817.5	2,603.1
Real Estate Revenues, Net									1,656.8	1,709.5	1,564.4
Operating expenses:
Compensation, operating and administrative expenses									2,670.9	2,689.7	2,507.9
Depreciation and amortization									43.1	50.8	44.6
Operating Expenses, Excluding Reimbursed Expenses									2,714.0	2,740.5	2,552.5
Gross contract vendor and subcontractor costs reimbursement									1,104.2	1,108.0	1,038.7
Total fee based segment operating expenses									1,609.8	1,632.5	1,513.8
General Contractor Cost									(1,104.2)	(1,108.0)	(1,038.7)
Operating Income (Loss)									47.0	77.0	50.6
Equity in earnings (losses)									(1.0)	0.0	0.3
Operating Income (Loss), Including Income (Loss) from Equity Method Investments									46.0	77.0	50.9
Segment Reconciling Items:
Operating Income (Loss)									47.0	77.0	50.6
Total Assets	2,658.4				2,325.4				2,658.4	2,325.4
Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues									1,377.2	1,369.2	1,347.9
Segment revenue:
Revenues									3,407.5	3,248.5	3,007.3
Revenue Before Reimbursements									2,030.3	1,879.3	1,659.4
Real Estate Revenues, Net									1,079.8	1,049.7	1,008.7
Operating expenses:
Compensation, operating and administrative expenses									1,876.6	1,743.0	1,520.1
Depreciation and amortization									26.4	23.2	22.1
Operating Expenses, Excluding Reimbursed Expenses									1,903.0	1,766.2	1,542.2
Gross contract vendor and subcontractor costs reimbursement									950.5	829.6	650.7
Total fee based segment operating expenses									952.5	936.6	891.5
General Contractor Cost									(950.5)	(829.6)	(650.7)
Operating Income (Loss)									127.3	113.1	117.2
Equity in earnings (losses)									2.3	2.0	3.2
Operating Income (Loss), Including Income (Loss) from Equity Method Investments									129.6	115.1	120.4
Segment Reconciling Items:
Operating Income (Loss)									127.3	113.1	117.2
Total Assets	1,642.8				1,437.3				1,642.8	1,437.3
LaSalle [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues									7.9	17.5	17.5
Segment revenue:
Revenues									526.1	533.9	355.8
Revenue Before Reimbursements									518.2	516.4	338.3
Real Estate Revenues, Net									502.9	511.9	333.2
Operating expenses:
Compensation, operating and administrative expenses									397.7	379.7	277.0
Depreciation and amortization									6.4	3.0	3.0
Operating Expenses, Excluding Reimbursed Expenses									404.1	382.7	280.0
Gross contract vendor and subcontractor costs reimbursement									15.3	4.5	5.1
Total fee based segment operating expenses									388.8	378.2	274.9
General Contractor Cost									(15.3)	(4.5)	(5.1)
Operating Income (Loss)									114.1	133.7	58.3
Equity in earnings (losses)									33.3	30.0	41.1
Operating Income (Loss), Including Income (Loss) from Equity Method Investments									147.4	163.7	99.4
Segment Reconciling Items:
Operating Income (Loss)									114.1	133.7	58.3
Total Assets	$ 774.5				$ 666.8				774.5	666.8
Leasing [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenues									1,960.5	1,754.1	1,510.1
Leasing [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenues									302.7	333.0	303.6
Leasing [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenues									260.8	285.0	238.2
Capital Markets [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenues									914.2	500.3	473.0
Capital Markets [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenues									411.9	464.1	465.1
Capital Markets [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenues									216.1	181.0	211.3
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenues									5,607.9	5,142.2	4,503.5
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenues									1,551.6	1,482.2	1,342.7
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenues									2,205.2	2,158.4	1,975.1
Project & Development Services [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenues									1,689.6	1,301.7	1,242.8
Project & Development Services [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenues									904.3	920.3	691.1
Project & Development Services [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenues									527.6	447.0	410.6
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenues									406.7	342.7	285.7
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenues									300.2	295.4	272.5
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenues									197.8	177.1	172.1
Advisory Fees [Member] \| LaSalle [Member]
Segment revenue:
Revenues									326.4	276.7	265.5
Transaction Fees & Other [Member] \| LaSalle [Member]
Segment revenue:
Revenues									61.3	41.4	33.4
Incentive Fees [Member] \| LaSalle [Member]
Segment revenue:
Revenues									$ 138.4	$ 215.8	$ 56.9