Revenue - Disaggregation of revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 1,895.4	$ 1,903.0
Americas
Disaggregation of Revenue [Line Items]
Revenue	1,394.8	1,347.6
EMEA
Disaggregation of Revenue [Line Items]
Revenue	210.0	202.6
APAC
Disaggregation of Revenue [Line Items]
Revenue	290.6	352.8
Property, facilities and project management | Over time
Disaggregation of Revenue [Line Items]
Revenue	1,301.2	1,232.9
Property, facilities and project management | Americas | Over time
Disaggregation of Revenue [Line Items]
Revenue	966.3	869.3
Property, facilities and project management | EMEA | Over time
Disaggregation of Revenue [Line Items]
Revenue	106.6	87.9
Property, facilities and project management | APAC | Over time
Disaggregation of Revenue [Line Items]
Revenue	228.3	275.7
Leasing | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	305.7	375.8
Leasing | Americas | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	242.1	300.1
Leasing | EMEA | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	40.8	48.9
Leasing | APAC | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	22.8	26.8
Capital markets | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	182.9	191.4
Capital markets | Americas | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	147.3	141.1
Capital markets | EMEA | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	21.6	26.4
Capital markets | APAC | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	14.0	23.9
Valuation and other | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue	105.6	102.9
Valuation and other | Americas | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue	39.1	37.1
Valuation and other | EMEA | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue	41.0	39.4
Valuation and other | APAC | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue	$ 25.5	$ 26.4