Segment Data - Schedule of summarized financial information by segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Total revenue	$ 2,076.0	$ 1,709.3	$ 5,818.0	$ 4,871.2
Less: Gross contract costs	(567.5)	(417.9)	(1,621.6)	(1,187.2)
Total Fee-based operating expenses	1,497.5	1,346.0	4,233.6	3,894.9
Adjusted EBITDA	179.0	102.2	423.6	261.9
Americas
Segment Reporting Information [Line Items]
Total revenue	1,475.6		4,053.9
EMEA
Segment Reporting Information [Line Items]
Total revenue	226.9		650.9
APAC
Segment Reporting Information [Line Items]
Total revenue	373.5		1,113.2
Operating segments
Segment Reporting Information [Line Items]
Total Fee-based operating expenses	1,329.6	1,190.9	3,779.2	3,437.3
Operating segments | Americas
Segment Reporting Information [Line Items]
Total revenue	1,475.6	1,137.4	4,053.9	3,253.3
Less: Gross contract costs	(424.9)	(255.4)	(1,164.2)	(726.8)
Acquisition accounting adjustments	0.0	0.4	2.5	13.0
Fee revenue	1,050.7	882.4	2,892.2	2,539.5
Segment operating expenses	1,346.6	1,061.4	3,742.1	3,064.7
Total Fee-based operating expenses	921.7	806.0	2,577.9	2,337.9
Adjusted EBITDA	128.8	76.3	314.2	201.3
Operating segments | EMEA
Segment Reporting Information [Line Items]
Total revenue	226.9	199.6	650.9	546.4
Less: Gross contract costs	(15.4)	(21.6)	(76.4)	(59.4)
Acquisition accounting adjustments	0.0	(0.1)	0.0	(0.1)
Fee revenue	211.5	177.9	574.5	486.9
Segment operating expenses	198.3	187.4	613.5	525.4
Total Fee-based operating expenses	182.9	165.8	537.1	466.0
Adjusted EBITDA	29.0	13.0	40.5	22.6
Operating segments | APAC
Segment Reporting Information [Line Items]
Total revenue	373.5	372.3	1,113.2	1,071.5
Less: Gross contract costs	(127.2)	(140.9)	(381.0)	(401.0)
Acquisition accounting adjustments	0.0	0.0	0.0	0.1
Fee revenue	246.3	231.4	732.2	670.6
Segment operating expenses	352.2	360.0	1,045.2	1,034.4
Total Fee-based operating expenses	225.0	219.1	664.2	633.4
Adjusted EBITDA	21.2	12.9	68.9	38.0
Operating segments | Property, facilities and project management | Americas
Segment Reporting Information [Line Items]
Fee revenue	427.4	408.9	1,257.8	1,206.6
Operating segments | Property, facilities and project management | EMEA
Segment Reporting Information [Line Items]
Fee revenue	60.0	45.7	178.1	132.3
Operating segments | Property, facilities and project management | APAC
Segment Reporting Information [Line Items]
Fee revenue	165.0	160.0	488.6	480.9
Operating segments | Leasing | Americas
Segment Reporting Information [Line Items]
Fee revenue	410.7	307.9	1,031.0	828.0
Operating segments | Leasing | EMEA
Segment Reporting Information [Line Items]
Fee revenue	67.2	54.8	173.9	154.0
Operating segments | Leasing | APAC
Segment Reporting Information [Line Items]
Fee revenue	41.4	35.7	110.4	92.2
Operating segments | Capital markets | Americas
Segment Reporting Information [Line Items]
Fee revenue	177.3	125.0	491.3	366.3
Operating segments | Capital markets | EMEA
Segment Reporting Information [Line Items]
Fee revenue	40.0	37.0	98.5	91.0
Operating segments | Capital markets | APAC
Segment Reporting Information [Line Items]
Fee revenue	16.5	12.8	59.1	35.2
Operating segments | Valuation and other | Americas
Segment Reporting Information [Line Items]
Fee revenue	35.3	40.6	112.1	138.6
Operating segments | Valuation and other | EMEA
Segment Reporting Information [Line Items]
Fee revenue	44.3	40.4	124.0	109.6
Operating segments | Valuation and other | APAC
Segment Reporting Information [Line Items]
Fee revenue	$ 23.4	$ 22.9	$ 74.1	$ 62.3