Business Segments - Segment Revenues (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment	Sep. 30, 2020 USD ($)
Segment Reporting [Abstract]
Number of operating segments | segment			2
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 3,946	$ 2,254	$ 9,411	$ 8,399
Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	3,627	1,945	8,589	7,670
Unallocated corporate and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	319	309	822	729
U.S. & Canada | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	3,006	1,586	7,055	6,485
International | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	621	359	1,534	1,185
Fee revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	776	397	1,863	1,260
Contract investment amortization	(21)	(48)	(56)	(94)
Revenues	755	349	1,807	1,166
Fee revenues | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	634	290	1,476	946
Contract investment amortization	(21)	(48)	(56)	(94)
Revenues	613	242	1,420	852
Fee revenues | U.S. & Canada | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	478	201	1,101	720
Contract investment amortization	(15)	(43)	(41)	(76)
Revenues	463	158	1,060	644
Fee revenues | International | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	156	89	375	226
Contract investment amortization	(6)	(5)	(15)	(18)
Revenues	150	84	360	208
Owned, leased, and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	241	116	536	445
Owned, leased, and other | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	227	99	505	405
Owned, leased, and other | U.S. & Canada | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	93	36	185	155
Owned, leased, and other | International | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	134	63	320	250
Reimbursements
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,950	1,789	7,068	6,788
Reimbursements | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,787	1,604	6,664	6,413
Reimbursements | U.S. & Canada | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	2,450	1,392	5,810	5,686
Reimbursements | International | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 337	$ 212	$ 854	$ 727