Business Segments - Segment Revenues (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) segment	Sep. 30, 2022 USD ($)
Segment Reporting [Abstract]
Number of operating segments | segment			2
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	$ 5,928	$ 5,313	$ 17,618	$ 14,850
Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	5,458	4,908	16,370	13,846
Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	4,333	4,000	13,113	11,388
Total segment | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	1,125	908	3,257	2,458
Unallocated corporate and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	470	405	1,248	1,004
Fee revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues	1,197	1,059	3,580	2,947
Contract investment amortization	(23)	(22)	(66)	(65)
Revenue	1,174	1,037	3,514	2,882
Fee revenues | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues	1,009	890	3,037	2,451
Contract investment amortization	(22)	(22)	(65)	(65)
Revenue	987	868	2,972	2,386
Fee revenues | Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues	690	649	2,113	1,821
Contract investment amortization	(16)	(16)	(49)	(45)
Revenue	674	633	2,064	1,776
Fee revenues | Total segment | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues	319	241	924	630
Contract investment amortization	(6)	(6)	(16)	(20)
Revenue	313	235	908	610
Owned, leased and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	363	345	1,109	971
Owned, leased and other | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	329	319	1,018	905
Owned, leased and other | Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	94	114	327	330
Owned, leased and other | Total segment | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	235	205	691	575
Reimbursements
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	4,391	3,931	12,995	10,997
Reimbursements | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	4,142	3,721	12,380	10,555
Reimbursements | Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	3,565	3,253	10,722	9,282
Reimbursements | Total segment | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	$ 577	$ 468	$ 1,658	$ 1,273