Business Segments (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Reimbursement Revenues	$ 1,859.0	$ 1,664.1
Segment revenue:
Revenue	3,820.6	3,555.2
Revenue Before Reimbursements	1,961.6	1,891.1
Operating expenses:
Depreciation and amortization	46.5	42.1
Operating Income (Loss)	25.4	53.8
Equity in earnings (losses)	5.0	13.6
Segment Reconciling Items:
Revenue	3,820.6	3,555.2
Total segment operating expenses before restructuring and acquisition charges	1,917.6	1,836.6
Cost of Reimbursable Expenses	1,859.0	1,664.1
Segment Operating Expenses before Restructuring and acquisition	3,776.6	3,500.7
Segment Reporting Information Operating Income Before Restructuring Charges	44.0	54.5
Restructuring and acquisition charges	18.6	0.7
Operating Income (Loss)	25.4	53.8
Assets	10,552.5		$ 10,025.5
Reportable Subsegments [Member]
Segment revenue:
Real Estate Revenues, Net	1,319.1	1,281.5
Operating expenses:
Gross contract vendor and subcontractor costs reimbursement	(642.6)	(606.9)
Reportable Subsegments [Member] \| Americas [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues	1,351.4	1,181.7
Segment revenue:
Revenue	2,250.1	1,941.0
Revenue Before Reimbursements	898.7	759.3
Derivative, Gain (Loss) on Derivative, Net	(0.1)	2.7
Real Estate Revenues, Net	711.1	623.5
Operating expenses:
Compensation, operating and administrative expenses	810.8	688.7
Depreciation and amortization	27.3	24.3
Operating Expenses, Excluding Reimbursed Expenses	838.1	713.0
Gross contract vendor and subcontractor costs reimbursement	(187.7)	(133.1)
Total fee based segment operating expenses	650.4	579.9
General Contractor Cost	187.7	133.1
Operating Income (Loss)	60.6	46.3
Equity in earnings (losses)	(0.3)	0.1
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	60.3	46.4
Segment Reconciling Items:
Revenue	2,250.1	1,941.0
Operating Income (Loss)	60.6	46.3
Reportable Subsegments [Member] \| EMEA [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues	164.6	156.0
Segment revenue:
Revenue	723.4	783.6
Revenue Before Reimbursements	558.8	627.6
Real Estate Revenues, Net	316.1	350.4
Operating expenses:
Compensation, operating and administrative expenses	576.9	636.2
Depreciation and amortization	11.3	11.4
Operating Expenses, Excluding Reimbursed Expenses	588.2	647.6
Gross contract vendor and subcontractor costs reimbursement	(242.7)	(277.2)
Total fee based segment operating expenses	345.5	370.4
General Contractor Cost	242.7	277.2
Operating Income (Loss)	(29.4)	(20.0)
Equity in earnings (losses)	0.1	0.0
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	(29.3)	(20.0)
Segment Reconciling Items:
Revenue	723.4	783.6
Operating Income (Loss)	(29.4)	(20.0)
Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues	341.1	321.6
Segment revenue:
Revenue	748.7	711.3
Revenue Before Reimbursements	407.6	389.7
Real Estate Revenues, Net	198.2	194.4
Operating expenses:
Compensation, operating and administrative expenses	400.5	385.0
Depreciation and amortization	6.4	5.6
Operating Expenses, Excluding Reimbursed Expenses	406.9	390.6
Gross contract vendor and subcontractor costs reimbursement	(209.4)	(195.3)
Total fee based segment operating expenses	197.5	195.3
General Contractor Cost	209.4	195.3
Operating Income (Loss)	0.7	(0.9)
Equity in earnings (losses)	0.3	0.3
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	1.0	(0.6)
Segment Reconciling Items:
Revenue	748.7	711.3
Operating Income (Loss)	0.7	(0.9)
Investment Management [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues	1.9	4.8
Segment revenue:
Revenue	98.4	119.3
Revenue Before Reimbursements	96.5	114.5
Real Estate Revenues, Net	93.7	113.2
Operating expenses:
Compensation, operating and administrative expenses	82.9	84.6
Depreciation and amortization	1.5	0.8
Operating Expenses, Excluding Reimbursed Expenses	84.4	85.4
Gross contract vendor and subcontractor costs reimbursement	(2.8)	(1.3)
Total fee based segment operating expenses	81.6	84.1
General Contractor Cost	2.8	1.3
Operating Income (Loss)	12.1	29.1
Equity in earnings (losses)	4.9	13.2
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	17.0	42.3
Segment Reconciling Items:
Revenue	98.4	119.3
Operating Income (Loss)	12.1	29.1
Leasing [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	389.8	304.5
Segment Reconciling Items:
Revenue	389.8	304.5
Leasing [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	52.2	58.9
Segment Reconciling Items:
Revenue	52.2	58.9
Leasing [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	35.9	37.8
Segment Reconciling Items:
Revenue	35.9	37.8
Capital Markets [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	100.1	111.0
Segment Reconciling Items:
Revenue	100.1	111.0
Capital Markets [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	64.0	89.3
Segment Reconciling Items:
Revenue	64.0	89.3
Capital Markets [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	29.4	32.4
Segment Reconciling Items:
Revenue	29.4	32.4
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	1,362.0	1,183.5
Segment Reconciling Items:
Revenue	1,362.0	1,183.5
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	369.3	350.3
Segment Reconciling Items:
Revenue	369.3	350.3
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	537.8	519.8
Segment Reconciling Items:
Revenue	537.8	519.8
Project & Development Services [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	308.7	269.6
Segment Reconciling Items:
Revenue	308.7	269.6
Project & Development Services [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	180.5	222.6
Segment Reconciling Items:
Revenue	180.5	222.6
Project & Development Services [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	110.9	87.0
Segment Reconciling Items:
Revenue	110.9	87.0
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	89.5	72.4
Segment Reconciling Items:
Revenue	89.5	72.4
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	57.4	62.5
Segment Reconciling Items:
Revenue	57.4	62.5
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	34.7	34.3
Segment Reconciling Items:
Revenue	34.7	34.3
Advisory Fees [Member] \| Investment Management [Member]
Segment revenue:
Revenue	77.6	69.9
Segment Reconciling Items:
Revenue	77.6	69.9
Transaction Fees & Other [Member] \| Investment Management [Member]
Segment revenue:
Revenue	13.2	16.7
Segment Reconciling Items:
Revenue	13.2	16.7
Incentive Fees [Member] \| Investment Management [Member]
Segment revenue:
Revenue	7.6	32.7
Segment Reconciling Items:
Revenue	$ 7.6	$ 32.7

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