Segments - Schedule of Summarized Financial Information by Segment (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting Information [Line Items]
Depreciation and amortization	$ 182	$ 161	$ 359	$ 319
Equity loss from unconsolidated subsidiaries	(18)	(15)	(2)	(73)
Operating Segments
Segment Reporting Information [Line Items]
Equity loss from unconsolidated subsidiaries	(18)	(15)	(2)	(73)
Operating Segments \| Advisory Services
Segment Reporting Information [Line Items]
Depreciation and amortization	67	60	134	126
Equity loss from unconsolidated subsidiaries	(1)	0	0	1
Operating Segments \| Building Operations & Experience
Segment Reporting Information [Line Items]
Depreciation and amortization	61	56	131	102
Equity loss from unconsolidated subsidiaries	(16)	3	(16)	4
Operating Segments \| Project Management
Segment Reporting Information [Line Items]
Depreciation and amortization	26	28	51	56
Equity loss from unconsolidated subsidiaries	0	0	0	0
Operating Segments \| Real Estate Investments
Segment Reporting Information [Line Items]
Depreciation and amortization	3	3	6	7
Equity loss from unconsolidated subsidiaries	(3)	4	(9)	15
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Depreciation and amortization	25	14	37	28
Equity loss from unconsolidated subsidiaries	$ 2	$ (22)	$ 23	$ (93)