LEASES - Future Minimum Lease Payments (Details) $ in Millions	Dec. 31, 2018 USD ($)
Aircraft Leases [Member]
Lessee, Lease, Description [Line Items]
Operating Leases, Future Minimum Payments Due, Next Twelve Months	$ 350
Operating Leases, Future Minimum Payments, Due in Rolling Year Two	320
Operating Leases, Future Minimum Payments, Due in Three Years	286
Operating Leases, Future Minimum Payments, Due in Rolling Year Four	262
Operating Leases, Future Minimum Payments, Due in Four Years	208
Thereafter	847
Operating Leases, Future Minimum Payments Due	2,273
Facility Lease [Member]
Lessee, Lease, Description [Line Items]
Operating Leases, Future Minimum Payments Due, Next Twelve Months	133
Operating Leases, Future Minimum Payments, Due in Rolling Year Two	124
Operating Leases, Future Minimum Payments, Due in Three Years	113
Operating Leases, Future Minimum Payments, Due in Rolling Year Four	94
Operating Leases, Future Minimum Payments, Due in Four Years	26
Thereafter	122
Operating Leases, Future Minimum Payments Due	$ 612