Business Segments (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment revenue:
Revenue	$ 3,555.2	$ 3,117.7
Reimbursement Revenue	1,664.1	1,501.4
Revenue Before Reimbursements	1,891.1	1,616.3
Operating expenses:
Depreciation and amortization	42.1	39.3
Total operating expenses	3,501.4	3,102.3
Operating income	53.8	15.4
Equity in earnings (losses)	13.6	5.6
Segment Reconciling Items:
Revenue	3,555.2	3,117.7
Total segment operating expenses before restructuring and acquisition charges	1,836.6	1,596.4
Cost of Reimbursable Expense	1,664.1	1,501.4
Segment Operating Expenses before Restructuring and acquisition	3,500.7	3,097.8
Segment Reporting Information Operating Income Before Restructuring Charges	54.5	19.9
Restructuring and acquisition charges	0.7	4.5
Operating income	53.8	15.4
Assets	9,204.2		$ 9,254.4
Reportable Subsegments [Member]
Segment revenue:
Total segment fee revenue	1,281.5	1,124.0
Operating expenses:
Gross contract vendor and subcontractor costs reimbursement	(606.9)	(495.0)
Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	1,941.0	1,799.9
Reimbursement Revenue	(1,181.7)	(1,078.5)
Revenue Before Reimbursements	759.3	721.4
Gross contract costs	(133.1)	(131.8)
Derivative, Gain (Loss) on Derivative, Net	2.7	(2.7)
Total segment fee revenue	623.5	592.3
Operating expenses:
Compensation, operating and administrative expenses	688.7	665.6
Depreciation and amortization	24.3	23.5
Total operating expenses	713.0	689.1
Gross contract vendor and subcontractor costs reimbursement	(133.1)	(131.8)
Total fee based segment operating expenses	579.9	557.3
Operating income	46.3	32.3
Equity in earnings (losses)	0.1	0.2
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	46.4	32.5
Segment Reconciling Items:
Revenue	1,941.0	1,799.9
Operating income	46.3	32.3
Assets	4,796.7		4,745.4
Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	783.6	603.2
Reimbursement Revenue	(156.0)	(101.9)
Revenue Before Reimbursements	627.6	501.3
Gross contract costs	(277.2)	(231.4)
Total segment fee revenue	350.4	269.9
Operating expenses:
Compensation, operating and administrative expenses	636.2	523.0
Depreciation and amortization	11.4	10.3
Total operating expenses	647.6	533.3
Gross contract vendor and subcontractor costs reimbursement	(277.2)	(231.4)
Total fee based segment operating expenses	370.4	301.9
Operating income	(20.0)	(32.0)
Equity in earnings (losses)	0.0	0.0
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	(20.0)	(32.0)
Segment Reconciling Items:
Revenue	783.6	603.2
Operating income	(20.0)	(32.0)
Assets	2,290.1		2,367.5
Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	711.3	626.3
Reimbursement Revenue	(321.6)	(316.5)
Revenue Before Reimbursements	389.7	309.8
Gross contract costs	(195.3)	(130.4)
Total segment fee revenue	194.4	179.4
Operating expenses:
Compensation, operating and administrative expenses	385.0	300.0
Depreciation and amortization	5.6	4.8
Total operating expenses	390.6	304.8
Gross contract vendor and subcontractor costs reimbursement	(195.3)	(130.4)
Total fee based segment operating expenses	195.3	174.4
Operating income	(0.9)	5.0
Equity in earnings (losses)	0.3	0.8
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	(0.6)	5.8
Segment Reconciling Items:
Revenue	711.3	626.3
Operating income	(0.9)	5.0
Assets	1,255.0		1,305.0
Investment Management [Member]
Segment revenue:
Revenue	119.3	88.3
Reimbursement Revenue	(4.8)	(4.5)
Revenue Before Reimbursements	114.5	83.8
Gross contract costs	(1.3)	(1.4)
Total segment fee revenue	113.2	82.4
Operating expenses:
Compensation, operating and administrative expenses	84.6	68.5
Depreciation and amortization	0.8	0.7
Total operating expenses	85.4	69.2
Gross contract vendor and subcontractor costs reimbursement	(1.3)	(1.4)
Total fee based segment operating expenses	84.1	67.8
Operating income	29.1	14.6
Equity in earnings (losses)	13.2	4.6
Operating Income (Loss), Including Income (Loss) from Equity Method Investments	42.3	19.2
Segment Reconciling Items:
Revenue	119.3	88.3
Operating income	29.1	14.6
Assets	549.5		548.6
Segment Reconciling Items [Member]
Segment Reconciling Items:
Assets	312.9		$ 287.9
Leasing [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	304.5	299.0
Segment Reconciling Items:
Revenue	304.5	299.0
Leasing [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	58.9	48.8
Segment Reconciling Items:
Revenue	58.9	48.8
Leasing [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	37.8	30.6
Segment Reconciling Items:
Revenue	37.8	30.6
Capital Markets [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	111.0	99.1
Segment Reconciling Items:
Revenue	111.0	99.1
Capital Markets [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	89.3	60.0
Segment Reconciling Items:
Revenue	89.3	60.0
Capital Markets [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	32.4	29.6
Segment Reconciling Items:
Revenue	32.4	29.6
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	1,183.5	1,049.1
Segment Reconciling Items:
Revenue	1,183.5	1,049.1
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	350.3	303.9
Segment Reconciling Items:
Revenue	350.3	303.9
Property & Facility Management [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	519.8	454.0
Segment Reconciling Items:
Revenue	519.8	454.0
Project & Development Services [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	269.6	290.5
Segment Reconciling Items:
Revenue	269.6	290.5
Project & Development Services [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	222.6	141.7
Segment Reconciling Items:
Revenue	222.6	141.7
Project & Development Services [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	87.0	77.7
Segment Reconciling Items:
Revenue	87.0	77.7
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| Americas [Member]
Segment revenue:
Revenue	72.4	62.2
Segment Reconciling Items:
Revenue	72.4	62.2
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| EMEA [Member]
Segment revenue:
Revenue	62.5	48.8
Segment Reconciling Items:
Revenue	62.5	48.8
Advisory, Consulting and Other [Member] \| Reportable Subsegments [Member] \| Asia Pacific [Member]
Segment revenue:
Revenue	34.3	34.4
Segment Reconciling Items:
Revenue	34.3	34.4
Advisory Fees [Member] \| Investment Management [Member]
Segment revenue:
Revenue	69.9	62.2
Segment Reconciling Items:
Revenue	69.9	62.2
Transaction Fees & Other [Member] \| Investment Management [Member]
Segment revenue:
Revenue	16.7	14.2
Segment Reconciling Items:
Revenue	16.7	14.2
Incentive Fees [Member] \| Investment Management [Member]
Segment revenue:
Revenue	32.7	11.9
Segment Reconciling Items:
Revenue	$ 32.7	$ 11.9

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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