Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Disaggregation of Revenue [Line Items]
Revenue	$ 2,331.0	$ 1,923.8
Americas
Disaggregation of Revenue [Line Items]
Revenue	1,785.3	1,424.9
EMEA
Disaggregation of Revenue [Line Items]
Revenue	237.7	223.9
APAC
Disaggregation of Revenue [Line Items]
Revenue	308.0	275.0
Property, facilities and project management | Over time
Disaggregation of Revenue [Line Items]
Revenue	1,460.2	1,350.2
Property, facilities and project management | Americas | Over time
Disaggregation of Revenue [Line Items]
Revenue	1,122.7	1,030.9
Property, facilities and project management | EMEA | Over time
Disaggregation of Revenue [Line Items]
Revenue	114.8	114.0
Property, facilities and project management | APAC | Over time
Disaggregation of Revenue [Line Items]
Revenue	222.7	205.3
Leasing | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	459.8	294.8
Leasing | Americas | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	372.9	222.8
Leasing | EMEA | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	49.7	42.6
Leasing | APAC | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	37.2	29.4
Capital markets | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	289.8	166.8
Capital markets | Americas | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	242.1	133.8
Capital markets | EMEA | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	28.9	22.3
Capital markets | APAC | At a point in time
Disaggregation of Revenue [Line Items]
Revenue	18.8	10.7
Valuation and other | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue	121.2	112.0
Valuation and other | Americas | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue	47.6	37.4
Valuation and other | EMEA | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue	44.3	45.0
Valuation and other | APAC | At a point in time or over time
Disaggregation of Revenue [Line Items]
Revenue	$ 29.3	$ 29.6