Revenue - Disaggregation of revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 1,743.6	$ 2,121.7	$ 3,639.0	$ 4,024.7
Americas
Disaggregation of Revenue [Line Items]
Revenue	1,248.5	1,492.9	2,643.3	2,840.5
EMEA
Disaggregation of Revenue [Line Items]
Revenue	205.1	228.9	415.1	431.5
APAC
Disaggregation of Revenue [Line Items]
Revenue	290.0	399.9	580.6	752.7
Property, facilities and project management | Over time
Disaggregation of Revenue [Line Items]
Revenue	1,262.6	1,280.8	2,563.8	2,513.7
Property, facilities and project management | Americas | Over time
Disaggregation of Revenue [Line Items]
Revenue	945.8	892.5	1,912.1	1,761.8
Property, facilities and project management | EMEA | Over time
Disaggregation of Revenue [Line Items]
Revenue	102.5	95.8	209.1	183.7
Property, facilities and project management | APAC | Over time
Disaggregation of Revenue [Line Items]
Revenue	214.3	292.5	442.6	568.2
Leasing | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	267.2	491.7	572.9	867.5
Leasing | Americas | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	193.4	389.0	435.5	689.1
Leasing | EMEA | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	41.5	57.0	82.3	105.9
Leasing | APAC | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	32.3	45.7	55.1	72.5
Capital markets | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	113.2	237.2	296.1	428.6
Capital markets | Americas | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	75.9	170.9	223.2	312.0
Capital markets | EMEA | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	24.0	33.2	45.6	59.6
Capital markets | APAC | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	13.3	33.1	27.3	57.0
Valuation and other | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue	100.6	112.0	206.2	214.9
Valuation and other | Americas | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue	33.4	40.5	72.5	77.6
Valuation and other | EMEA | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue	37.1	42.9	78.1	82.3
Valuation and other | APAC | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue	$ 30.1	$ 28.6	$ 55.6	$ 55.0