Organization (Details) $ in Millions, ft² in Billions	12 Months Ended
Dec. 31, 2020 USD ($) ft² employee	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of employees \| employee	91,000
Number of employees whose costs are reimbursed by clients \| employee	41,100
Portfolio of property and corporate facility management (in square feet) \| ft²	5.4
Assets under management by LaSalle Investment Management	$ 68,900.0
Product Information [Line Items]
Revenue:	16,589.9	$ 17,983.2	$ 16,318.4
Real Estate Services: Leasing [Member]
Product Information [Line Items]
Revenue:	1,884.2	2,524.0	2,372.1
Real Estate Services: Capital Markets and Hotels [Member]
Product Information [Line Items]
Revenue:	1,407.4	1,542.2	1,145.4
Real Estate Services: Property & Facilities Management [Member]
Product Information [Line Items]
Revenue:	9,485.4	9,364.7	8,782.8
Real Estate Services: Project & Development Services [Member]
Product Information [Line Items]
Revenue:	2,530.1	3,121.5	2,669.0
Real Estate Services: Advisory, Consulting and Other [Member]
Product Information [Line Items]
Revenue:	861.0	904.7	815.2
LaSalle Investment Management [Member]
Product Information [Line Items]
Revenue:	$ 421.8	$ 526.1	$ 533.9