Business Segments (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Reimbursement Revenues	$ 1,863.0	$ 1,859.0
Segment revenue:
Revenue	4,096.0	3,820.6
Revenue Before Reimbursements	2,233.0	1,961.6
Operating expenses:
Depreciation and amortization	55.0	46.5
Operating Income (Loss)	64.6	25.4
Equity in earnings (losses)	(28.3)	5.0
Segment Reconciling Items:
Revenue	4,096.0	3,820.6
Total segment operating expenses before restructuring and acquisition charges	2,154.3	1,917.6
Cost of Reimbursable Expenses	1,863.0	1,859.0
Segment Operating Expenses before Restructuring and acquisition	4,017.3	3,776.6
Segment Reporting Information Operating Income Before Restructuring Charges	78.7	44.0
Restructuring and acquisition charges	14.1	18.6
Operating Income (Loss)	64.6	25.4
Assets	13,900.8		$ 13,672.6
Reportable Subsegments [Member]
Segment revenue:
Real Estate Revenues, Net	1,505.2	1,319.1
Operating expenses:
Gross contract vendor and subcontractor costs reimbursement	729.4	642.6
Reportable Subsegments [Member] \| Americas [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues	1,393.5	1,351.4
Segment revenue:
Revenue	2,523.1	2,250.1
Revenue Before Reimbursements	1,129.6	898.7
Derivative, Gain (Loss) on Derivative, Net	(1.6)	(0.1)
Real Estate Revenues, Net	918.4	711.1
Operating expenses:
Compensation, operating and administrative expenses	1,010.3	810.8
Depreciation and amortization	37.4	27.3
Operating Expenses, Excluding Reimbursed Expenses	1,047.7	838.1
Gross contract vendor and subcontractor costs reimbursement	212.8	187.7
Total fee based segment operating expenses	834.9	650.4
General Contractor Cost	(212.8)	(187.7)
Operating Income (Loss)	81.9	60.6
Equity in earnings (losses)	12.7	(0.3)
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	94.6	60.3
Segment Reconciling Items:
Revenue	2,523.1	2,250.1
Operating Income (Loss)	81.9	60.6
Reportable Subsegments [Member] \| EMEA [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues	182.8	164.6
Segment revenue:
Revenue	755.9	723.4
Revenue Before Reimbursements	573.1	558.8
Real Estate Revenues, Net	310.5	316.1
Operating expenses:
Compensation, operating and administrative expenses	584.4	576.9
Depreciation and amortization	9.2	11.3
Operating Expenses, Excluding Reimbursed Expenses	593.6	588.2
Gross contract vendor and subcontractor costs reimbursement	262.6	242.7
Total fee based segment operating expenses	331.0	345.5
General Contractor Cost	(262.6)	(242.7)
Operating Income (Loss)	(20.5)	(29.4)
Equity in earnings (losses)	0.0	0.1
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	(20.5)	(29.3)
Segment Reconciling Items:
Revenue	755.9	723.4
Operating Income (Loss)	(20.5)	(29.4)
Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues	284.9	341.1
Segment revenue:
Revenue	712.1	748.7
Revenue Before Reimbursements	427.2	407.6
Real Estate Revenues, Net	177.7	198.2
Operating expenses:
Compensation, operating and administrative expenses	417.5	400.5
Depreciation and amortization	6.6	6.4
Operating Expenses, Excluding Reimbursed Expenses	424.1	406.9
Gross contract vendor and subcontractor costs reimbursement	249.5	209.4
Total fee based segment operating expenses	174.6	197.5
General Contractor Cost	(249.5)	(209.4)
Operating Income (Loss)	3.1	0.7
Equity in earnings (losses)	(0.7)	0.3
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	2.4	1.0
Segment Reconciling Items:
Revenue	712.1	748.7
Operating Income (Loss)	3.1	0.7
Investment Management [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues	1.8	1.9
Segment revenue:
Revenue	104.9	98.4
Revenue Before Reimbursements	103.1	96.5
Real Estate Revenues, Net	98.6	93.7
Operating expenses:
Compensation, operating and administrative expenses	87.1	82.9
Depreciation and amortization	1.8	1.5
Operating Expenses, Excluding Reimbursed Expenses	88.9	84.4
Gross contract vendor and subcontractor costs reimbursement	4.5	2.8
Total fee based segment operating expenses	84.4	81.6
General Contractor Cost	(4.5)	(2.8)
Operating Income (Loss)	14.2	12.1
Equity in earnings (losses)	(40.3)	4.9
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	(26.1)	17.0
Segment Reconciling Items:
Revenue	104.9	98.4
Operating Income (Loss)	14.2	12.1
Leasing [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	418.9	389.8
Segment Reconciling Items:
Revenue	418.9	389.8
Leasing [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	48.1	52.2
Segment Reconciling Items:
Revenue	48.1	52.2
Leasing [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	25.4	35.9
Segment Reconciling Items:
Revenue	25.4	35.9
Capital Markets [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	247.4	100.1
Segment Reconciling Items:
Revenue	247.4	100.1
Capital Markets [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	73.2	64.0
Segment Reconciling Items:
Revenue	73.2	64.0
Capital Markets [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	21.7	29.4
Segment Reconciling Items:
Revenue	21.7	29.4
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	1,458.8	1,362.0
Segment Reconciling Items:
Revenue	1,458.8	1,362.0
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	375.0	369.3
Segment Reconciling Items:
Revenue	375.0	369.3
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	532.0	537.8
Segment Reconciling Items:
Revenue	532.0	537.8
Project & Development Services [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	306.4	308.7
Segment Reconciling Items:
Revenue	306.4	308.7
Project & Development Services [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	203.2	180.5
Segment Reconciling Items:
Revenue	203.2	180.5
Project & Development Services [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	94.8	110.9
Segment Reconciling Items:
Revenue	94.8	110.9
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	91.6	89.5
Segment Reconciling Items:
Revenue	91.6	89.5
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	56.4	57.4
Segment Reconciling Items:
Revenue	56.4	57.4
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	38.2	34.7
Segment Reconciling Items:
Revenue	38.2	34.7
Advisory Fees [Member] \| Investment Management [Member]
Segment revenue:
Revenue	85.6	77.6
Segment Reconciling Items:
Revenue	85.6	77.6
Transaction Fees & Other [Member] \| Investment Management [Member]
Segment revenue:
Revenue	13.6	13.2
Segment Reconciling Items:
Revenue	13.6	13.2
Incentive Fees [Member] \| Investment Management [Member]
Segment revenue:
Revenue	5.7	7.6
Segment Reconciling Items:
Revenue	$ 5.7	$ 7.6

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