Business Segments (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Reimbursement Revenues	$ 1,756.1	$ 1,541.5	$ 5,160.6	$ 4,638.7
Segment revenue:
Revenue	3,969.8	3,522.2	11,428.7	10,110.2
Revenue Before Reimbursements	2,213.7	1,980.7	6,268.1	5,471.5
Operating expenses:
Depreciation and amortization	42.7	41.8	131.1	122.3
Operating Income (Loss)	190.9	137.8	394.5	284.0
Equity in earnings (losses)	3.4	12.6	27.2	32.7
Segment Reconciling Items:
Revenue	3,969.8	3,522.2	11,428.7	10,110.2
Total segment operating expenses before restructuring and acquisition charges	2,019.1	1,839.5	5,880.3	5,174.2
Cost of Reimbursable Expenses	1,756.1	1,541.5	5,160.6	4,638.7
Segment Operating Expenses before Restructuring and acquisition	3,775.2	3,381.0	11,040.9	9,812.9
Segment Reporting Information Operating Income Before Restructuring Charges	194.6	141.2	387.8	297.3
Restructuring and acquisition charges	3.7	3.4	(6.7)	13.3
Operating Income (Loss)	190.9	137.8	394.5	284.0
Assets	9,619.1		9,619.1		$ 9,254.4
Reportable Subsegments [Member]
Segment revenue:
Real Estate Revenues, Net	1,591.4	1,419.5	4,366.4	3,867.8
Operating expenses:
Gross contract vendor and subcontractor costs reimbursement	(617.0)	(554.1)	(1,892.4)	(1,592.6)
Reportable Subsegments [Member] \| Americas [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues	1,251.5	1,094.9	3,659.6	3,283.4
Segment revenue:
Revenue	2,201.6	1,920.6	6,295.1	5,653.0
Revenue Before Reimbursements	950.1	825.7	2,635.5	2,369.6
Derivative, Gain (Loss) on Derivative, Net	5.3	7.1	9.3	11.1
Real Estate Revenues, Net	774.1	693.7	2,165.8	1,971.4
Operating expenses:
Compensation, operating and administrative expenses	821.2	710.2	2,306.8	2,078.2
Depreciation and amortization	24.5	24.2	77.1	71.4
Operating Expenses, Excluding Reimbursed Expenses	845.7	734.4	2,383.9	2,149.6
Gross contract vendor and subcontractor costs reimbursement	(170.7)	(124.9)	(460.4)	(387.1)
Total fee based segment operating expenses	675.0	609.5	1,923.5	1,762.5
General Contractor Cost	170.7	124.9	460.4	387.1
Operating Income (Loss)	104.4	91.3	251.6	220.0
Equity in earnings (losses)	0.1	0.1	0.6	0.5
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	104.5	91.4	252.2	220.5
Segment Reconciling Items:
Revenue	2,201.6	1,920.6	6,295.1	5,653.0
Operating Income (Loss)	104.4	91.3	251.6	220.0
Assets	5,292.8		5,292.8		4,745.4
Reportable Subsegments [Member] \| EMEA [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues	171.0	108.5	480.0	335.9
Segment revenue:
Revenue	805.6	750.0	2,435.8	2,074.4
Revenue Before Reimbursements	634.6	641.5	1,955.8	1,738.5
Real Estate Revenues, Net	396.7	382.6	1,135.4	999.3
Operating expenses:
Compensation, operating and administrative expenses	611.7	625.9	1,931.4	1,723.5
Depreciation and amortization	11.8	11.6	34.7	33.0
Operating Expenses, Excluding Reimbursed Expenses	623.5	637.5	1,966.1	1,756.5
Gross contract vendor and subcontractor costs reimbursement	(237.9)	(258.9)	(820.4)	(739.2)
Total fee based segment operating expenses	385.6	378.6	1,145.7	1,017.3
General Contractor Cost	237.9	258.9	820.4	739.2
Operating Income (Loss)	11.1	4.0	(10.3)	(18.0)
Equity in earnings (losses)	0.0	0.0	0.0	0.0
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	11.1	4.0	(10.3)	(18.0)
Segment Reconciling Items:
Revenue	805.6	750.0	2,435.8	2,074.4
Operating Income (Loss)	11.1	4.0	(10.3)	(18.0)
Assets	2,168.3		2,168.3		2,367.5
Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues	330.0	333.5	1,007.8	1,005.9
Segment revenue:
Revenue	790.0	749.4	2,314.2	2,119.4
Revenue Before Reimbursements	460.0	415.9	1,306.4	1,113.5
Real Estate Revenues, Net	252.3	246.8	698.0	651.0
Operating expenses:
Compensation, operating and administrative expenses	432.6	386.5	1,245.1	1,047.2
Depreciation and amortization	5.6	5.2	17.0	15.7
Operating Expenses, Excluding Reimbursed Expenses	438.2	391.7	1,262.1	1,062.9
Gross contract vendor and subcontractor costs reimbursement	(207.7)	(169.1)	(608.4)	(462.5)
Total fee based segment operating expenses	230.5	222.6	653.7	600.4
General Contractor Cost	207.7	169.1	608.4	462.5
Operating Income (Loss)	21.8	24.2	44.3	50.6
Equity in earnings (losses)	1.0	0.9	2.0	2.3
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	22.8	25.1	46.3	52.9
Segment Reconciling Items:
Revenue	790.0	749.4	2,314.2	2,119.4
Operating Income (Loss)	21.8	24.2	44.3	50.6
Assets	1,287.9		1,287.9		1,305.0
Investment Management [Member]
Segment Reporting Information [Line Items]
Reimbursement Revenues	3.6	4.6	13.2	13.5
Segment revenue:
Revenue	172.6	102.2	383.6	263.4
Revenue Before Reimbursements	169.0	97.6	370.4	249.9
Real Estate Revenues, Net	168.3	96.4	367.2	246.1
Operating expenses:
Compensation, operating and administrative expenses	110.9	75.1	265.9	203.0
Depreciation and amortization	0.8	0.8	2.3	2.2
Operating Expenses, Excluding Reimbursed Expenses	111.7	75.9	268.2	205.2
Gross contract vendor and subcontractor costs reimbursement	(0.7)	(1.2)	(3.2)	(3.8)
Total fee based segment operating expenses	111.0	74.7	265.0	201.4
General Contractor Cost	0.7	1.2	3.2	3.8
Operating Income (Loss)	57.3	21.7	102.2	44.7
Equity in earnings (losses)	2.3	11.6	24.6	29.9
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	59.6	33.3	126.8	74.6
Segment Reconciling Items:
Revenue	172.6	102.2	383.6	263.4
Operating Income (Loss)	57.3	21.7	102.2	44.7
Assets	584.3		584.3		548.6
Segment Reconciling Items [Member]
Segment Reconciling Items:
Assets	285.8		285.8		$ 287.9
Leasing [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	424.0	370.1	1,151.4	1,057.5
Segment Reconciling Items:
Revenue	424.0	370.1	1,151.4	1,057.5
Leasing [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	80.1	71.0	211.9	186.6
Segment Reconciling Items:
Revenue	80.1	71.0	211.9	186.6
Leasing [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	69.3	51.9	165.2	133.7
Segment Reconciling Items:
Revenue	69.3	51.9	165.2	133.7
Capital Markets [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	115.6	115.4	344.4	319.2
Segment Reconciling Items:
Revenue	115.6	115.4	344.4	319.2
Capital Markets [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	108.9	107.2	288.1	263.7
Segment Reconciling Items:
Revenue	108.9	107.2	288.1	263.7
Capital Markets [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	39.5	55.7	122.2	121.5
Segment Reconciling Items:
Revenue	39.5	55.7	122.2	121.5
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	1,271.5	1,077.0	3,713.4	3,230.7
Segment Reconciling Items:
Revenue	1,271.5	1,077.0	3,713.4	3,230.7
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	336.3	335.9	1,078.9	975.0
Segment Reconciling Items:
Revenue	336.3	335.9	1,078.9	975.0
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	529.2	490.0	1,590.6	1,449.4
Segment Reconciling Items:
Revenue	529.2	490.0	1,590.6	1,449.4
Project & Development Services [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	304.7	288.2	848.8	843.7
Segment Reconciling Items:
Revenue	304.7	288.2	848.8	843.7
Project & Development Services [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	220.1	170.3	663.5	472.0
Segment Reconciling Items:
Revenue	220.1	170.3	663.5	472.0
Project & Development Services [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	108.7	109.5	309.8	297.4
Segment Reconciling Items:
Revenue	108.7	109.5	309.8	297.4
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	85.8	69.9	237.1	201.9
Segment Reconciling Items:
Revenue	85.8	69.9	237.1	201.9
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	60.2	65.6	193.4	177.1
Segment Reconciling Items:
Revenue	60.2	65.6	193.4	177.1
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	43.3	42.3	126.4	117.4
Segment Reconciling Items:
Revenue	43.3	42.3	126.4	117.4
Advisory Fees [Member] \| Investment Management [Member]
Segment revenue:
Revenue	68.0	63.4	205.0	190.0
Segment Reconciling Items:
Revenue	68.0	63.4	205.0	190.0
Transaction Fees & Other [Member] \| Investment Management [Member]
Segment revenue:
Revenue	9.8	5.6	33.2	24.9
Segment Reconciling Items:
Revenue	9.8	5.6	33.2	24.9
Incentive Fees [Member] \| Investment Management [Member]
Segment revenue:
Revenue	94.8	33.2	145.4	48.5
Segment Reconciling Items:
Revenue	$ 94.8	$ 33.2	$ 145.4	$ 48.5

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