Revenue - Disaggregation of revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 2,076.0	$ 1,709.3	$ 5,818.0	$ 4,871.2
Americas
Disaggregation of Revenue [Line Items]
Revenue	1,475.6		4,053.9
EMEA
Disaggregation of Revenue [Line Items]
Revenue	226.9		650.9
APAC
Disaggregation of Revenue [Line Items]
Revenue	373.5		1,113.2
Property, facilities and project management | Over time
Disaggregation of Revenue [Line Items]
Revenue	1,216.5		3,534.2
Property, facilities and project management | Americas | Over time
Disaggregation of Revenue [Line Items]
Revenue	849.3		2,412.0
Property, facilities and project management | EMEA | Over time
Disaggregation of Revenue [Line Items]
Revenue	75.0		252.6
Property, facilities and project management | APAC | Over time
Disaggregation of Revenue [Line Items]
Revenue	292.2		869.6
Leasing | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	522.0		1,319.5
Leasing | Americas | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	413.1		1,034.6
Leasing | EMEA | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	67.5		174.5
Leasing | APAC | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	41.4		110.4
Capital markets | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	234.3		650.4
Capital markets | Americas | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	177.8		492.8
Capital markets | EMEA | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	40.0		98.5
Capital markets | APAC | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	16.5		59.1
Valuation and other | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue	103.2		313.9
Valuation and other | Americas | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue	35.4		114.5
Valuation and other | EMEA | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue	44.4		125.3
Valuation and other | APAC | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue	$ 23.4		$ 74.1