Revenue - Disaggregation of revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 2,607.5	$ 2,118.8	$ 2,121.7	$ 1,903.0	$ 2,401.9	$ 2,076.0	$ 1,974.3	$ 1,767.7	$ 8,751.0	$ 8,219.9	$ 6,923.9
Revenue before adoption											6,923.9
Americas
Disaggregation of Revenue [Line Items]
Revenue									6,172.1	5,724.7
Revenue before adoption											4,600.2
EMEA
Disaggregation of Revenue [Line Items]
Revenue									1,038.2	999.8
Revenue before adoption											863.3
APAC
Disaggregation of Revenue [Line Items]
Revenue									1,540.7	1,495.4
Revenue before adoption											1,460.4
Property, facilities and project management | Over time
Disaggregation of Revenue [Line Items]
Revenue									5,277.0	4,877.5
Revenue before adoption											4,101.1
Property, facilities and project management | Americas | Over time
Disaggregation of Revenue [Line Items]
Revenue									3,723.2	3,369.6
Revenue before adoption											2,650.3
Property, facilities and project management | EMEA | Over time
Disaggregation of Revenue [Line Items]
Revenue									412.8	371.1
Revenue before adoption											278.6
Property, facilities and project management | APAC | Over time
Disaggregation of Revenue [Line Items]
Revenue									1,141.0	1,136.8
Revenue before adoption											1,172.2
Leasing | At a point in time
Disaggregation of Revenue [Line Items]
Revenue									1,950.8	1,927.7
Revenue before adoption											1,635.9
Leasing | Americas | At a point in time
Disaggregation of Revenue [Line Items]
Revenue									1,519.0	1,487.5
Revenue before adoption											1,229.3
Leasing | EMEA | At a point in time
Disaggregation of Revenue [Line Items]
Revenue									251.5	266.1
Revenue before adoption											256.9
Leasing | APAC | At a point in time
Disaggregation of Revenue [Line Items]
Revenue									180.3	174.1
Revenue before adoption											149.7
Capital markets | At a point in time
Disaggregation of Revenue [Line Items]
Revenue									1,033.7	962.7
Revenue before adoption											741.1
Capital markets | Americas | At a point in time
Disaggregation of Revenue [Line Items]
Revenue									746.9	702.4
Revenue before adoption											531.4
Capital markets | EMEA | At a point in time
Disaggregation of Revenue [Line Items]
Revenue									182.2	173.6
Revenue before adoption											153.9
Capital markets | APAC | At a point in time
Disaggregation of Revenue [Line Items]
Revenue									104.6	86.7
Revenue before adoption											55.8
Valuation and other | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue									489.5	452.0
Revenue before adoption											445.8
Valuation and other | Americas | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue									183.0	165.2
Revenue before adoption											189.2
Valuation and other | EMEA | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue									191.7	189.0
Revenue before adoption											173.9
Valuation and other | APAC | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue									$ 114.8	$ 97.8
Revenue before adoption											$ 82.7