COMMITMENTS AND CONTINGENCIES - Commitments (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) lawsuit
Commitments and Contingencies Disclosure [Line Items]
Loan commitment	$ 14
Loan commitment funded in 2019	5
Loan commitment expected to be funded thereafter	5
Letters of credit outstanding	136
Surety bonds issued	152
Information Technology Hardware, Software, Accounting, Finance, and Maintenance Services
Commitments and Contingencies Disclosure [Line Items]
Purchase obligation	286
Purchase obligation, expected to fund commitments in 2019	153
Purchase obligation, expected to fund commitments in 2020	78
Purchase obligation, expected to fund commitments in 2021	38
Purchase obligation, thereafter	17
Commitments
Commitments and Contingencies Disclosure [Line Items]
Commitments, funding not expected	33
Data Security Incident
Commitments and Contingencies Disclosure [Line Items]
Expense related to data security incident	$ 28
Insurance recoveries related to data security incident	25
Start-Up Specializing in Evaluation of Travel and Real Estate Related Emerging Technology Companies | Investment in Non-controlling Interest in Partnership
Commitments and Contingencies Disclosure [Line Items]
Investment commitment	$ 11
Investment commitment expected to be funded in 2019	3
Investment commitment expected to be funded thereafter	$ 6
Putative Class Action Lawsuits Related to Data Security Incident
Commitments and Contingencies Disclosure [Line Items]
Loss Contingency, Pending Claims, Number | lawsuit	100