Segments - Summarized Financial Information by Segment (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Revenue	$ 31,949.0	$ 30,828.0	$ 27,746.0
Depreciation and amortization	622.0	613.0	526.0
Equity income from unconsolidated subsidiaries	248.0	229.0	619.0
Segment Operating Profit	1,117.0	1,512.0	1,637.0
Amortization of right-to-use assets	36.0	31.0
Operating Segments
Segment Reporting Information [Line Items]
Segment Operating Profit	2,609.0	3,327.0	3,291.0
Corporate And Reconciling Items
Segment Reporting Information [Line Items]
Revenue	(17.0)	(16.0)	(20.0)
Depreciation and amortization	56.0	33.0	29.0
Equity income from unconsolidated subsidiaries	27.0	(167.0)	37.0
Segment Operating Profit	368.0	578.0	217.0
Advisory Services \| Operating Segments
Segment Reporting Information [Line Items]
Revenue	8,499.0	9,883.0	9,575.0
Depreciation and amortization	289.0	311.0	311.0
Equity income from unconsolidated subsidiaries	4.0	15.0	25.0
Segment Operating Profit	1,364.0	1,910.0	2,063.0
Global Workplace Solutions \| Operating Segments
Segment Reporting Information [Line Items]
Revenue	22,515.0	19,851.0	17,099.0
Depreciation and amortization	262.0	253.0	159.0
Equity income from unconsolidated subsidiaries	1.0	1.0	2.0
Segment Operating Profit	1,006.0	899.0	708.0
Amortization of right-to-use assets	46.4	52.7	52.2
Real Estate Investments \| Operating Segments
Segment Reporting Information [Line Items]
Revenue	952.0	1,110.0	1,092.0
Depreciation and amortization	15.0	16.0	27.0
Equity income from unconsolidated subsidiaries	216.0	380.0	555.0
Segment Operating Profit	$ 239.0	$ 518.0	$ 520.0

The current period expense charged against earnings on long-lived, physical assets not used in production, and which are not intended for resale, to allocate or recognize the cost of such assets over their useful lives; or to record the reduction in book value of an intangible asset over the benefit period of such asset; or to reflect consumption during the period of an asset that is not used in production.

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