SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details)		1 Months Ended	3 Months Ended								12 Months Ended
		May 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	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 ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2020 USD ($)
Significant Accounting Policies [Line Items]
Revenues			$ 2,172,000,000	$ 2,254,000,000	$ 1,464,000,000	$ 4,681,000,000	$ 5,371,000,000	$ 5,284,000,000	$ 5,305,000,000	$ 5,012,000,000	$ 10,571,000,000	$ 20,972,000,000	$ 20,758,000,000
Compensation costs from retirement savings plan											75,000,000	128,000,000	224,000,000
Advertising costs											276,000,000	851,000,000	660,000,000
Goodwill impairment charge											0	0	0
Self-insurance reserve, current			121,000,000				166,000,000				121,000,000	166,000,000
Self-insurance reserve, noncurrent			341,000,000				323,000,000				341,000,000	323,000,000
Allowance for credit losses			207,000,000				82,000,000				207,000,000	82,000,000			$ 101,000,000
Deferred tax liabilities			83,000,000				290,000,000				83,000,000	290,000,000
Equity			430,000,000				703,000,000				430,000,000	703,000,000	2,225,000,000	$ 3,582,000,000
Change in allowance for credit losses											136,000,000
Disposal Group, Disposed of by Sale, Not Discontinued Operations
Significant Accounting Policies [Line Items]
Gain (loss) on disposal													132,000,000
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Avendra LLC
Significant Accounting Policies [Line Items]
Gain (loss) on disposal														664,000,000
Gain (loss) on disposal, net of tax														425,000,000
Retained Earnings
Significant Accounting Policies [Line Items]
Equity			$ 9,206,000,000				9,644,000,000				$ 9,206,000,000	9,644,000,000	8,982,000,000	$ 7,242,000,000
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-13
Significant Accounting Policies [Line Items]
Allowance for credit losses															19,000,000
Deferred tax liabilities															(4,000,000)
Equity							(15,000,000)					(15,000,000)
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-13 | Retained Earnings
Significant Accounting Policies [Line Items]
Equity							(15,000,000)					(15,000,000)			$ 15,000,000
Measurement Input, Discount Rate | Valuation Technique, Discounted Cash Flow
Significant Accounting Policies [Line Items]
Self insurance review, measurement input			0.02								0.02
Deferred revenue
Significant Accounting Policies [Line Items]
Change in customer contract liability											$ 907,000,000
Customer contract liability			$ 1,867,000,000				960,000,000				1,867,000,000	960,000,000
Guest loyalty program
Significant Accounting Policies [Line Items]
Change in customer contract liability											553,000,000
Customer contract liability			6,271,000,000				5,718,000,000				6,271,000,000	5,718,000,000
Revenue recognized from customer contract liability											1,118,000,000
Change in customer contract liability from change in estimates											(47,000,000)
Change in customer contract liability from prepayment		$ 920,000,000
Reimbursements
Significant Accounting Policies [Line Items]
Revenues	[1]										8,452,000,000	15,599,000,000	15,543,000,000
Reimbursements | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Avendra LLC
Significant Accounting Policies [Line Items]
Revenues											62,000,000	118,000,000	115,000,000
Revenues, net of tax											46,000,000	87,000,000	$ 85,000,000
Costs Incurred to Fulfill Contracts
Significant Accounting Policies [Line Items]
Capitalized costs			$ 366,000,000				$ 351,000,000				$ 366,000,000	$ 351,000,000

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