BUSINESS SEGMENTS - Segment Revenues (Details) $ in Millions		12 Months Ended
		Dec. 31, 2022 USD ($) segment	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Segment Reporting, Revenue Reconciling Item [Line Items]
Number of reporting segments | segment		2
Total revenue		$ 20,773	$ 13,857	$ 10,571
Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue		19,239	12,610	9,502
Total segment | Non-US
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue		4,032	2,615	1,910
Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue		15,753	10,356	7,905
Total segment | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue		3,486	2,254	1,597
Unallocated corporate and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue		1,534	1,247	1,069
Net fee revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues		4,078	2,694	1,683
Contract investment amortization		(89)	(75)	(132)
Total revenue		3,989	2,619	1,551
Net fee revenues | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues		3,403	2,148	1,252
Contract investment amortization		(89)	(75)	(132)
Total revenue		3,314	2,073	1,120
Net fee revenues | Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues		2,486	1,580	914
Contract investment amortization		(60)	(55)	(108)
Total revenue		2,426	1,525	806
Net fee revenues | Total segment | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues		917	568	338
Contract investment amortization		(29)	(20)	(24)
Total revenue		888	548	314
Owned, leased, and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue		1,367	796	568
Owned, leased, and other | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue		1,280	749	515
Owned, leased, and other | Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue		479	282	198
Owned, leased, and other | Total segment | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue		801	467	317
Reimbursements
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue	[1]	15,417	10,442	8,452
Reimbursements | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue		14,645	9,788	7,867
Reimbursements | Total segment | Non-US
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue		2,231	1,553	1,247
Reimbursements | Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue		12,848	8,549	6,901
Reimbursements | Total segment | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue		$ 1,797	$ 1,239	$ 966

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