Business Segments - Segment Revenues (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2022 USD ($) segment	Jun. 30, 2021 USD ($)
Segment Reporting [Abstract]
Number of operating segments | segment			2
Segment Reporting, Revenue Reconciling Item [Line Items]
Number of operating segments | segment			2
Revenues	$ 5,338	$ 3,149	$ 9,537	$ 5,465
Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	4,992	2,850	8,938	4,962
Unallocated corporate and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	346	299	599	503
U.S. & Canada | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	4,117	2,328	7,388	4,049
International | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	875	522	1,550	913
Fee revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	1,073	642	1,888	1,087
Contract investment amortization	(19)	(18)	(43)	(35)
Revenues	1,054	624	1,845	1,052
Fee revenues | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	895	505	1,561	842
Contract investment amortization	(19)	(18)	(43)	(35)
Revenues	876	487	1,518	807
Fee revenues | U.S. & Canada | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	683	373	1,172	623
Contract investment amortization	(15)	(13)	(29)	(26)
Revenues	668	360	1,143	597
Fee revenues | International | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	212	132	389	219
Contract investment amortization	(4)	(5)	(14)	(9)
Revenues	208	127	375	210
Owned, leased, and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	364	187	626	295
Owned, leased, and other | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	341	177	586	278
Owned, leased, and other | U.S. & Canada | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	124	57	216	92
Owned, leased, and other | International | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	217	120	370	186
Reimbursements
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	3,920	2,338	7,066	4,118
Reimbursements | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	3,775	2,186	6,834	3,877
Reimbursements | U.S. & Canada | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	3,325	1,911	6,029	3,360
Reimbursements | International | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 450	$ 275	$ 805	$ 517