BUSINESS SEGMENTS - Summary of Segment Revenue, Segment Expenses, and Segment Profits (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenue		$ 25,100	$ 23,713	$ 20,773
Depreciation, amortization, and other		183	189	193
General, administrative, and other		1,074	1,011	891
OPERATING INCOME		3,767	3,864	3,462
Net fee revenues
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Gross fee revenues		5,170	4,824	4,078
Contract investment amortization		(103)	(88)	(89)
Total revenue		5,067	4,736	3,989
Owned, leased, and other
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenue		1,551	1,564	1,367
Cost of revenues		1,200	1,165	1,074
Reimbursements
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenue	[1]	18,482	17,413	15,417
Cost of revenues	[1]	18,799	17,424	15,141
Total segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenue		22,576	21,394	18,597
Total segment | U.S. & Canada
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenue		18,612	17,696	15,753
Depreciation, amortization, and other		84	84	81
General, administrative, and other		182	154	135
Other segment items		(5)	(5)	(9)
OPERATING INCOME		2,640	2,724	2,446
Total segment | EMEA
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenue		2,406	2,268	1,929
Depreciation, amortization, and other		38	40	45
General, administrative, and other		110	140	103
Other segment items		(5)	(2)	(2)
OPERATING INCOME		512	441	435
Total segment | Greater China
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenue		582	600	351
Depreciation, amortization, and other		9	10	12
General, administrative, and other		55	55	50
Other segment items		(1)	(2)	3
OPERATING INCOME		186	208	62
Total segment | APEC
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenue		976	830	564
Depreciation, amortization, and other		8	9	9
General, administrative, and other		64	61	48
Other segment items		(4)	(8)	(7)
OPERATING INCOME		280	243	134
Total segment | Net fee revenues | U.S. & Canada
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Gross fee revenues		2,951	2,799	2,486
Contract investment amortization		(76)	(65)	(60)
Total revenue		2,875	2,734	2,426
Total segment | Net fee revenues | EMEA
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Gross fee revenues		589	529	455
Contract investment amortization		(14)	(13)	(17)
Total revenue		575	516	438
Total segment | Net fee revenues | Greater China
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Gross fee revenues		250	265	120
Contract investment amortization		(1)	0	0
Total revenue		249	265	120
Total segment | Net fee revenues | APEC
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Gross fee revenues		345	288	181
Contract investment amortization		(5)	(4)	(3)
Total revenue		340	284	178
Total segment | Owned, leased, and other | U.S. & Canada
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenue		437	506	479
Cost of revenues		330	340	386
Total segment | Owned, leased, and other | EMEA
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenue		595	574	481
Cost of revenues		499	481	369
Total segment | Owned, leased, and other | Greater China
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenue		27	18	17
Cost of revenues		14	12	14
Total segment | Owned, leased, and other | APEC
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenue		141	135	97
Cost of revenues		118	116	99
Total segment | Reimbursements | U.S. & Canada
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenue		15,300	14,456	12,848
Cost of revenues		15,381	14,399	12,714
Total segment | Reimbursements | EMEA
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenue		1,236	1,178	1,010
Cost of revenues		1,252	1,168	979
Total segment | Reimbursements | Greater China
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenue		306	317	214
Cost of revenues		319	317	210
Total segment | Reimbursements | APEC
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenue		495	411	289
Cost of revenues		$ 510	$ 409	$ 281

[1] See Note 15 for disclosure of related party amounts.