Segments - Summarized Financial Information by Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenue	$ 6,910,501	$ 5,645,142	$ 5,381,384	$ 5,889,168	$ 7,119,407	$ 5,925,101	$ 5,714,073	$ 5,135,510	$ 23,826,195	$ 23,894,091	$ 21,340,088
Depreciation and amortization									501,728	439,224	451,988
Equity income from unconsolidated subsidiaries									126,161	160,925	324,664
Adjusted EBITDA									1,892,385	2,063,783	1,905,168
Advisory Services
Segment Reporting Information [Line Items]
Revenue									7,698,992	9,069,468	8,440,002
Depreciation and amortization									348,669	304,766	280,921
Equity income from unconsolidated subsidiaries									2,245	6,894	16,017
Adjusted EBITDA									1,143,889	1,465,792	1,303,251
Global Workspace Solutions
Segment Reporting Information [Line Items]
Revenue									15,295,673	14,164,001	12,365,362
Depreciation and amortization									125,692	120,975	147,222
Equity income from unconsolidated subsidiaries									368	(1,423)	115
Adjusted EBITDA									516,814	424,026	345,560
Real Estate Investments
Segment Reporting Information [Line Items]
Revenue									831,530	660,622	534,724
Depreciation and amortization									27,367	13,483	23,845
Equity income from unconsolidated subsidiaries									123,548	155,454	308,532
Adjusted EBITDA									$ 231,682	$ 173,965	$ 256,357