BUSINESS SEGMENTS - Segment Revenues (Details) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($) segment	Mar. 31, 2023 USD ($)
Segment Reporting [Abstract]
Number of reporting segments | segment	4
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	$ 5,977	$ 5,615
Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	5,383	5,073
Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	4,490	4,278
Total segment | EMEA
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	511	471
Total segment | Greater China
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	148	131
Total segment | APEC
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	234	193
Unallocated corporate and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	594	542
Fee revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues	1,210	1,133
Contract investment amortization	(23)	(21)
Revenue	1,187	1,112
Fee revenues | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues	952	901
Contract investment amortization	(21)	(20)
Revenue	931	881
Fee revenues | Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues	682	672
Contract investment amortization	(17)	(16)
Revenue	665	656
Fee revenues | Total segment | EMEA
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues	118	105
Contract investment amortization	(3)	(3)
Revenue	115	102
Fee revenues | Total segment | Greater China
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues	65	57
Contract investment amortization	0	0
Revenue	65	57
Fee revenues | Total segment | APEC
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues	87	67
Contract investment amortization	(1)	(1)
Revenue	86	66
Owned, leased and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	357	356
Owned, leased and other | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	265	264
Owned, leased and other | Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	108	117
Owned, leased and other | Total segment | EMEA
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	118	113
Owned, leased and other | Total segment | Greater China
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	7	4
Owned, leased and other | Total segment | APEC
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	32	30
Reimbursements
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	4,433	4,147
Reimbursements | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	4,187	3,928
Reimbursements | Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	3,717	3,505
Reimbursements | Total segment | EMEA
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	278	256
Reimbursements | Total segment | Greater China
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	76	70
Reimbursements | Total segment | APEC
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	$ 116	$ 97