BUSINESS SEGMENTS - Segment Revenues (Details) $ in Millions		3 Months Ended									12 Months Ended
		Mar. 31, 2021 segment	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting, Revenue Reconciling Item [Line Items]
Number of reporting segments | segment											3
Total revenue			$ 2,172	$ 2,254	$ 1,464	$ 4,681	$ 5,371	$ 5,284	$ 5,305	$ 5,012	$ 10,571	$ 20,972	$ 20,758
Forecast
Segment Reporting, Revenue Reconciling Item [Line Items]
Number of reporting segments | segment		2
Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue											9,275	19,954	19,276
Total segment | International Operations
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue											1,910	4,400	4,246
Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue											7,905	16,833	16,289
Total segment | Asia Pacific
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue											612	1,189	1,118
Total segment | EMEA
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue											758	1,932	1,869
Unallocated corporate and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue											1,296	1,018	1,482
Net fee revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues											1,683	3,823	3,638
Contract investment amortization											(132)	(62)	(58)
Total revenue											1,551	3,761	3,580
Net fee revenues | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues											1,207	3,160	3,024
Contract investment amortization											(127)	(58)	(54)
Total revenue											1,080	3,102	2,970
Net fee revenues | Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues											914	2,265	2,158
Contract investment amortization											(108)	(48)	(45)
Total revenue											806	2,217	2,113
Net fee revenues | Total segment | Asia Pacific
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues											183	477	479
Contract investment amortization											(7)	(2)	(2)
Total revenue											176	475	477
Net fee revenues | Total segment | EMEA
Segment Reporting, Revenue Reconciling Item [Line Items]
Gross fee revenues											110	418	387
Contract investment amortization											(12)	(8)	(7)
Total revenue											98	410	380
Owned, leased, and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue											568	1,612	1,635
Owned, leased, and other | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue											440	1,446	1,466
Owned, leased, and other | Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue											198	715	721
Owned, leased, and other | Total segment | Asia Pacific
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue											89	178	182
Owned, leased, and other | Total segment | EMEA
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue											153	553	563
Reimbursements
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue	[1]										8,452	15,599	15,543
Reimbursements | Total segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue											7,755	15,406	14,840
Reimbursements | Total segment | International Operations
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue											1,247	2,394	2,244
Reimbursements | Total segment | U.S. & Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue											6,901	13,901	13,455
Reimbursements | Total segment | Asia Pacific
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue											347	536	459
Reimbursements | Total segment | EMEA
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenue											$ 507	$ 969	$ 926

[1] See Note 16 for disclosure of related party amounts.