Goodwill and Other Intangible Assets - Schedule of Changes in Goodwill by Segment (Details) - USD ($)		3 Months Ended	12 Months Ended
	Jan. 01, 2021	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Roll Forward]
goodwill, gross, beginning balance	$ 4,915,115,000	$ 4,821,999,000	$ 4,915,115,000	$ 4,821,999,000
Accumulated impairment losses			(1,093,506,000)	(1,093,506,000)	$ (1,068,506,000)
Goodwill, beginning balance	3,821,609,000	3,753,493,000	3,821,609,000	3,753,493,000
Reallocation			0
Purchase accounting entries related to acquisitions			1,245,294,000	18,181,000
Goodwill impairment	0	(25,000,000)	0	(25,000,000)
Foreign exchange movement			(71,728,000)	74,935,000
Goodwill, gross, ending balance			6,088,681,000	4,915,115,000
Goodwill, ending balance			4,995,175,000	3,821,609,000
Advisory Services
Goodwill [Roll Forward]
goodwill, gross, beginning balance	3,348,788,000	3,302,218,000	3,348,788,000	3,302,218,000
Accumulated impairment losses			(761,448,000)	(761,448,000)	(761,448,000)
Goodwill, beginning balance	2,587,340,000	2,540,770,000	2,587,340,000	2,540,770,000
Reallocation			(101,390,000)
Purchase accounting entries related to acquisitions			77,616,000	16,463,000
Goodwill impairment			0	0
Foreign exchange movement			(26,520,000)	30,107,000
Goodwill, gross, ending balance			3,298,494,000	3,348,788,000
Goodwill, ending balance			2,537,046,000	2,587,340,000
Global Workplace Solutions
Goodwill [Roll Forward]
goodwill, gross, beginning balance	937,797,000	899,506,000	937,797,000	899,506,000
Accumulated impairment losses			(175,473,000)	(175,473,000)	(175,473,000)
Goodwill, beginning balance	762,324,000	724,033,000	762,324,000	724,033,000
Reallocation	101,400,000		101,390,000
Purchase accounting entries related to acquisitions			1,167,678,000	9,702,000
Goodwill impairment			0	0
Foreign exchange movement			(32,836,000)	28,589,000
Goodwill, gross, ending balance			2,174,029,000	937,797,000
Goodwill, ending balance			1,998,556,000	762,324,000
Real Estate Investments
Goodwill [Roll Forward]
goodwill, gross, beginning balance	628,530,000	620,275,000	628,530,000	620,275,000
Accumulated impairment losses			(156,585,000)	(156,585,000)	$ (131,585,000)
Goodwill, beginning balance	$ 471,945,000	$ 488,690,000	471,945,000	488,690,000
Reallocation			0
Purchase accounting entries related to acquisitions			0	(7,984,000)
Goodwill impairment			0	(25,000,000)
Foreign exchange movement			(12,372,000)	16,239,000
Goodwill, gross, ending balance			616,158,000	628,530,000
Goodwill, ending balance			$ 459,573,000	$ 471,945,000