LEASES - NARRATIVE (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) aircraft	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Lessee, Lease, Description [Line Items]
Sublease Income	$ 14	$ 13
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	81
Operating Lease, Payments	302
Rent expense			$ 619
Lessor, Operating Lease, Lease Not yet Commenced, Assumption and Judgment, Value of Underlying Asset, Amount	$ 453
Property Subject to Operating Lease [Member] | Airbus
Lessee, Lease, Description [Line Items]
Operating leases, number of leased assets (in aircraft) | aircraft	71
Property Subject to Operating Lease [Member] | Q400
Lessee, Lease, Description [Line Items]
Operating leases, number of leased assets (in aircraft) | aircraft	7
Property Subject to Operating Lease [Member] | E175 [Member]
Lessee, Lease, Description [Line Items]
Operating leases, number of leased assets (in aircraft) | aircraft	32
Property Subject to Operating Lease [Member] | B737
Lessee, Lease, Description [Line Items]
Operating leases, number of leased assets (in aircraft) | aircraft	10
Airport Facilities [Member]
Lessee, Lease, Description [Line Items]
Variable Lease, Cost	$ 286	322
Aircraft Leases [Member] | Maximum [Member]
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Term of Contract	11 years
Corporate real estate [Member]
Lessee, Lease, Description [Line Items]
Variable Lease, Cost	$ 12	$ 10
CPA Aircraft [Member] | Maximum [Member]
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Term of Contract	10 years
CPA Aircraft [Member] | Minimum [Member]
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Term of Contract	6 years 6 months