Segments - Schedule of Summarized Financial Information by Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Revenue	$ 9,036	$ 7,868	$ 25,363	$ 22,999
Operating income	368	269	819	613
Operating Segments
Segment Reporting Information [Line Items]
Operating income	799	535	1,939	1,747
Operating Segments | Advisory Services
Segment Reporting Information [Line Items]
Revenue	2,395	2,013	6,518	5,908
Operating income	414	277	1,020	862
Operating Segments | Global Workplace Solutions
Segment Reporting Information [Line Items]
Revenue	6,346	5,649	18,099	16,413
Operating income	318	251	808	714
Operating Segments | Real Estate Investments
Segment Reporting Information [Line Items]
Revenue	302	210	762	690
Operating income	67	7	111	171
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Revenue	(7)	(4)	(16)	(12)
Operating income	$ 119	$ 111	$ 412	$ 319