BUSINESS SEGMENTS - Segment Revenues (Details) $ in Millions	3 Months Ended
	Mar. 31, 2023 USD ($) segment	Mar. 31, 2022 USD ($)
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 5,615	$ 4,199
Number of operating segments | segment	2
Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 5,286	3,946
Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	4,278	3,271
Total segment | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,008	675
Unallocated corporate and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	329	253
Fee revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	1,133	815
Contract investment amortization	(21)	(24)
Revenues	1,112	791
Fee revenues | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	963	666
Contract investment amortization	(21)	(24)
Revenues	942	642
Fee revenues | Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	672	489
Contract investment amortization	(16)	(14)
Revenues	656	475
Fee revenues | Total segment | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	291	177
Contract investment amortization	(5)	(10)
Revenues	286	167
Owned, leased, and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	356	262
Owned, leased, and other | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	331	245
Owned, leased, and other | Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	117	92
Owned, leased, and other | Total segment | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	214	153
Reimbursements
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	4,147	3,146
Reimbursements | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	4,013	3,059
Reimbursements | Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	3,505	2,704
Reimbursements | Total segment | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 508	$ 355