Business Segments - Segment Revenues (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 2,254	$ 5,284	$ 8,399	$ 15,601
Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,905	4,953	7,490	14,840
Unallocated corporate and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	349	331	909	761
North America | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,586	4,168	6,485	12,546
Asia Pacific | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	157	285	412	865
EMEA | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	162	500	593	1,429
Fee revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	397	955	1,260	2,849
Contract investment amortization	(48)	(16)	(94)	(45)
Revenues	349	939	1,166	2,804
Fee revenues | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	283	797	913	2,361
Contract investment amortization	(47)	(16)	(91)	(43)
Revenues	236	781	822	2,318
Fee revenues | North America | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	201	570	720	1,711
Contract investment amortization	(43)	(13)	(76)	(35)
Revenues	158	557	644	1,676
Fee revenues | Asia Pacific | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	55	114	112	344
Contract investment amortization	(1)	(1)	(6)	(2)
Revenues	54	113	106	342
Fee revenues | EMEA | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross revenues	27	113	81	306
Contract investment amortization	(3)	(2)	(9)	(6)
Revenues	24	111	72	300
Owned, leased, and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	116	393	445	1,186
Owned, leased, and other | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	84	352	341	1,072
Owned, leased, and other | North America | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	36	171	155	534
Owned, leased, and other | Asia Pacific | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	22	39	60	130
Owned, leased, and other | EMEA | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	26	142	126	408
Reimbursements
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,789	3,952	6,788	11,611
Reimbursements | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,585	3,820	6,327	11,450
Reimbursements | North America | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	1,392	3,440	5,686	10,336
Reimbursements | Asia Pacific | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	81	133	246	393
Reimbursements | EMEA | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 112	$ 247	$ 395	$ 721