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Business Segments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
Business Segments (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting [Abstract]
Percentage of assets from countries having restrictions									6.00%	6.00%
Segment Reporting Information [Line Items]
Reimbursement Revenues									$ 7,228.9	$ 6,485.8	$ 6,080.6
Segment revenue:
Revenues	$ 4,889.7	$ 3,969.8	$ 3,903.7	$ 3,555.2	$ 4,343.0	$ 3,522.2	$ 3,470.3	$ 3,117.7	16,318.4	14,453.2	12,991.2
Revenue Before Reimbursements									9,089.5	7,967.4	6,910.6
Segment Reporting Information Operating Expenses									8,343.8	7,390.8	6,386.4
Operating expenses:
Depreciation and amortization									186.1	167.2	141.8
Operating Income (Loss)	312.4	190.9	149.8	53.8	261.9	137.8	130.8	15.4	706.9	545.9	455.7
Equity in earnings (losses)									32.8	44.4	33.8
Segment Reconciling Items:
Segment Operating Expenses before Restructuring and acquisition									15,572.7	13,876.6	12,467.0
Segment Reporting Information Operating Income Before Restructuring Charges									745.7	576.6	524.2
Restructuring and acquisition charges	45.5	3.7	11.1	0.7	17.4	3.4	5.4	4.5	38.8	30.7	68.5
Operating Income (Loss)	312.4	$ 190.9	$ 149.8	$ 53.8	261.9	$ 137.8	$ 130.8	$ 15.4	706.9	545.9	455.7
Total Assets	10,025.5				9,254.4				10,025.5	9,254.4
Reportable Subsegments [Member]
Segment revenue:
Real Estate Revenues, Net									6,486.2	5,736.2	5,074.3
Operating expenses:
Gross contract vendor and subcontractor costs reimbursement									2,595.0	2,215.5	1,812.8
Reportable Subsegments [Member] \| Americas [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues									5,164.7	4,648.5	4,460.1
Segment revenue:
Revenues									9,041.0	8,015.1	7,516.8
Revenue Before Reimbursements									3,876.3	3,366.6	3,056.7
Derivative, Gain (Loss) on Derivative, Net									(8.3)	(15.7)	(23.5)
Real Estate Revenues, Net									3,215.1	2,829.9	2,479.2
Operating expenses:
Compensation, operating and administrative expenses									3,345.3	2,918.6	2,686.4
Depreciation and amortization									109.1	97.5	84.3
Operating Expenses, Excluding Reimbursed Expenses									3,454.4	3,016.1	2,770.7
Gross contract vendor and subcontractor costs reimbursement									652.9	521.0	554.0
Total fee based segment operating expenses									2,801.5	2,495.1	2,216.7
General Contractor Cost									(652.9)	(521.0)	(554.0)
Operating Income (Loss)									421.9	350.5	286.0
Equity in earnings (losses)									0.8	(0.2)	1.3
Operating Income (Loss), Including Income (Loss) from Equity Method Investments									422.7	350.3	287.3
Segment Reconciling Items:
Operating Income (Loss)									421.9	350.5	286.0
Total Assets	5,288.9				4,745.4				5,288.9	4,745.4
Reportable Subsegments [Member] \| EMEA [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues									677.5	471.9	352.0
Segment revenue:
Revenues									3,495.0	3,075.0	2,447.6
Revenue Before Reimbursements									2,817.5	2,603.1	2,095.6
Real Estate Revenues, Net									1,709.5	1,564.4	1,337.9
Operating expenses:
Compensation, operating and administrative expenses									2,689.7	2,507.9	1,992.5
Depreciation and amortization									50.8	44.6	37.4
Operating Expenses, Excluding Reimbursed Expenses									2,740.5	2,552.5	2,029.9
Gross contract vendor and subcontractor costs reimbursement									1,108.0	1,038.7	757.7
Total fee based segment operating expenses									1,632.5	1,513.8	1,272.2
General Contractor Cost									(1,108.0)	(1,038.7)	(757.7)
Operating Income (Loss)									77.0	50.6	65.7
Equity in earnings (losses)									0.0	0.3	(0.1)
Operating Income (Loss), Including Income (Loss) from Equity Method Investments									77.0	50.9	65.6
Segment Reconciling Items:
Operating Income (Loss)									77.0	50.6	65.7
Total Assets	2,325.4				2,367.5				2,325.4	2,367.5
Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues									1,369.2	1,347.9	1,248.3
Segment revenue:
Revenues									3,248.5	3,007.3	2,618.7
Revenue Before Reimbursements									1,879.3	1,659.4	1,370.4
Real Estate Revenues, Net									1,049.7	1,008.7	874.7
Operating expenses:
Compensation, operating and administrative expenses									1,743.0	1,520.1	1,264.2
Depreciation and amortization									23.2	22.1	17.3
Operating Expenses, Excluding Reimbursed Expenses									1,766.2	1,542.2	1,281.5
Gross contract vendor and subcontractor costs reimbursement									829.6	650.7	495.7
Total fee based segment operating expenses									936.6	891.5	785.8
General Contractor Cost									(829.6)	(650.7)	(495.7)
Operating Income (Loss)									113.1	117.2	88.9
Equity in earnings (losses)									2.0	3.2	1.1
Operating Income (Loss), Including Income (Loss) from Equity Method Investments									115.1	120.4	90.0
Segment Reconciling Items:
Operating Income (Loss)									113.1	117.2	88.9
Total Assets	1,437.3				1,305.0				1,437.3	1,305.0
LaSalle [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues									17.5	17.5	20.2
Segment revenue:
Revenues									533.9	355.8	408.1
Revenue Before Reimbursements									516.4	338.3	387.9
Real Estate Revenues, Net									511.9	333.2	382.5
Operating expenses:
Compensation, operating and administrative expenses									379.7	277.0	301.5
Depreciation and amortization									3.0	3.0	2.8
Operating Expenses, Excluding Reimbursed Expenses									382.7	280.0	304.3
Gross contract vendor and subcontractor costs reimbursement									4.5	5.1	5.4
Total fee based segment operating expenses									378.2	274.9	298.9
General Contractor Cost									(4.5)	(5.1)	(5.4)
Operating Income (Loss)									133.7	58.3	83.6
Equity in earnings (losses)									30.0	41.1	31.5
Operating Income (Loss), Including Income (Loss) from Equity Method Investments									163.7	99.4	115.1
Segment Reconciling Items:
Operating Income (Loss)									133.7	58.3	83.6
Total Assets	$ 666.8				$ 548.6				666.8	548.6
Leasing [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenues									1,754.1	1,510.1	1,311.5
Leasing [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenues									333.0	303.6	270.0
Leasing [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenues									285.0	238.2	216.9
Capital Markets [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenues									500.3	473.0	428.0
Capital Markets [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenues									464.1	465.1	390.0
Capital Markets [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenues									181.0	211.3	162.1
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenues									5,142.2	4,503.5	4,475.0
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenues									1,482.2	1,342.7	881.1
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenues									2,158.4	1,975.1	1,780.4
Project & Development Services [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenues									1,301.7	1,242.8	1,104.4
Project & Development Services [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenues									920.3	691.1	659.7
Project & Development Services [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenues									447.0	410.6	306.2
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenues									342.7	285.7	197.9
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenues									295.4	272.5	246.8
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenues									177.1	172.1	153.1
Advisory Fees [Member] \| LaSalle [Member]
Segment revenue:
Revenues									276.7	265.5	260.4
Transaction Fees & Other [Member] \| LaSalle [Member]
Segment revenue:
Revenues									41.4	33.4	51.8
Incentive Fees [Member] \| LaSalle [Member]
Segment revenue:
Revenues									$ 215.8	$ 56.9	$ 95.9