LEASES - Lease Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Operating Lease, Expense	$ 602	$ 668
Sublease Income	14	13
Lease, Cost	588	655
Aircraft Leases [Member]
Lessee, Lease, Description [Line Items]
Operating Lease, Cost	$ 215	246
Operating Lease, Weighted Average Discount Rate, Percent	3.90%
Operating Lease, Weighted Average Remaining Lease Term	6 years 4 months 24 days
CPA Aircraft [Member]
Lessee, Lease, Description [Line Items]
Operating Lease, Cost	$ 80	79
Operating Lease, Weighted Average Discount Rate, Percent	2.70%
Operating Lease, Weighted Average Remaining Lease Term	8 years 2 months 12 days
Airport Facilities [Member]
Lessee, Lease, Description [Line Items]
Operating Lease, Cost	$ 288	324
Operating Lease, Weighted Average Discount Rate, Percent	4.10%
Operating Lease, Weighted Average Remaining Lease Term	9 years 3 months 18 days
Corporate real estate [Member]
Lessee, Lease, Description [Line Items]
Operating Lease, Cost	$ 19	$ 19
Operating Lease, Weighted Average Discount Rate, Percent	4.10%
Operating Lease, Weighted Average Remaining Lease Term	31 years 7 months 6 days