BUSINESS SEGMENTS - Segment Revenues (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) segment	Sep. 30, 2023 USD ($)
Segment Reporting [Abstract]
Number of reporting segments \| segment			4
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	$ 6,255	$ 5,928	$ 18,671	$ 17,618
Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	5,603	5,295	16,752	15,811
Total segment \| U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	4,596	4,333	13,851	13,113
Total segment \| EMEA
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	635	603	1,775	1,662
Total segment \| Greater China
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	142	160	430	445
Total segment \| APEC
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	230	199	696	591
Unallocated corporate and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	652	633	1,919	1,807
Net fee revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues	1,283	1,197	3,836	3,580
Contract investment amortization	(26)	(23)	(76)	(66)
Revenue	1,257	1,174	3,760	3,514
Net fee revenues \| Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues	1,044	973	3,081	2,889
Contract investment amortization	(24)	(21)	(71)	(62)
Revenue	1,020	952	3,010	2,827
Net fee revenues \| Total segment \| U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues	747	690	2,227	2,113
Contract investment amortization	(19)	(16)	(57)	(49)
Revenue	728	674	2,170	2,064
Net fee revenues \| Total segment \| EMEA
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues	153	143	425	381
Contract investment amortization	(3)	(4)	(10)	(10)
Revenue	150	139	415	371
Net fee revenues \| Total segment \| Greater China
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues	62	71	186	196
Contract investment amortization	0	0	0	0
Revenue	62	71	186	196
Net fee revenues \| Total segment \| APEC
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues	82	69	243	199
Contract investment amortization	(2)	(1)	(4)	(3)
Revenue	80	68	239	196
Owned, leased, and other revenue
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	381	363	1,133	1,109
Owned, leased, and other revenue \| Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	299	283	874	856
Owned, leased, and other revenue \| Total segment \| U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	95	94	314	327
Owned, leased, and other revenue \| Total segment \| EMEA
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	169	155	444	419
Owned, leased, and other revenue \| Total segment \| Greater China
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	5	6	18	15
Owned, leased, and other revenue \| Total segment \| APEC
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	30	28	98	95
Cost reimbursement revenue
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	4,617	4,391	13,778	12,995
Cost reimbursement revenue \| Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	4,284	4,060	12,868	12,128
Cost reimbursement revenue \| Total segment \| U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	3,773	3,565	11,367	10,722
Cost reimbursement revenue \| Total segment \| EMEA
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	316	309	916	872
Cost reimbursement revenue \| Total segment \| Greater China
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	75	83	226	234
Cost reimbursement revenue \| Total segment \| APEC
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	$ 120	$ 103	$ 359	$ 300

Revenues, Gross Of Capitalized Contract Cost, Amortization

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Amount of amortization expense for asset recognized from cost incurred to obtain or fulfill contract with customer.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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