Business Segments - Segment Revenues (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($) segment	Sep. 30, 2021 USD ($)
Segment Reporting [Abstract]
Number of operating segments | segment			2
Segment Reporting, Revenue Reconciling Item [Line Items]
Number of operating segments | segment			2
Revenues	$ 5,313	$ 3,946	$ 14,850	$ 9,411
Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	4,908	3,627	13,846	8,589
Unallocated corporate and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	405	319	1,004	822
U.S. & Canada | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	4,000	3,006	11,388	7,055
International | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	908	621	2,458	1,534
Fee revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	1,059	776	2,947	1,863
Contract investment amortization	(22)	(21)	(65)	(56)
Revenues	1,037	755	2,882	1,807
Fee revenues | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	890	634	2,451	1,476
Contract investment amortization	(22)	(21)	(65)	(56)
Revenues	868	613	2,386	1,420
Fee revenues | U.S. & Canada | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	649	478	1,821	1,101
Contract investment amortization	(16)	(15)	(45)	(41)
Revenues	633	463	1,776	1,060
Fee revenues | International | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	241	156	630	375
Contract investment amortization	(6)	(6)	(20)	(15)
Revenues	235	150	610	360
Owned, leased, and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	345	241	971	536
Owned, leased, and other | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	319	227	905	505
Owned, leased, and other | U.S. & Canada | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	114	93	330	185
Owned, leased, and other | International | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	205	134	575	320
Reimbursements
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	3,931	2,950	10,997	7,068
Reimbursements | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	3,721	2,787	10,555	6,664
Reimbursements | U.S. & Canada | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	3,253	2,450	9,282	5,810
Reimbursements | International | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 468	$ 337	$ 1,273	$ 854