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Acquisitions - Narrative (Details) $ in Millions		9 Months Ended
Acquisitions - Narrative (Details) $ in Millions	Jan. 16, 2025 USD ($)	Sep. 30, 2025 USD ($)	Sep. 30, 2025 USD ($) acquisition	Sep. 30, 2025 USD ($) business	Jan. 01, 2025
Business Combination [Line Items]
Number of businesses acquired			2	1
Advisory Services
Business Combination [Line Items]
Number of businesses acquired \| business				1
Project Management \| Turner & Townsend
Business Combination [Line Items]
Equity method investments in unconsolidated subsidiaries, variations in ownership percentage					70.00%
Industrious
Business Combination [Line Items]
Equity interest percentage	60.00%
Goodwill deductible amount		$ 392	$ 392	$ 392
Cash and deferred consideration	$ 369
Industrious \| Global Workplace Solutions
Business Combination [Line Items]
Equity interest percentage	100.00%
Turner & Townsend \| Project Management
Business Combination [Line Items]
Equity interest percentage					10.00%
Estimated based on discounted forecasted cash flows		409	$ 409	$ 409
Other Acquisitions
Business Combination [Line Items]
Cash and deferred consideration		$ 31