INCOME TAXES - DEFERRED TAX ASSETS AND LIABILITIES (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Excess of tax over book depreciation	$ 1,126	$ 1,233
Deferred Tax Liabilities, Intangible Assets	15	16
Deferred Tax Liabilities, Other	106	58
Gross deferred tax liabilities	1,589	1,723
Mileage Plan	(385)	(337)
Inventory obsolescence	(17)	(15)
Employee benefits	(215)	(179)
Net operating losses	(27)	(13)
Other - net	(103)	(48)
Deferred tax assets	(1,201)	(1,009)
Valuation allowance	19	1
Net deferred tax liabilities	407	715
Operating Loss Carryforwards [Line Items]
Deferred Tax Liabilities, Other Finite-Lived Assets	342	416
Deferred Tax Assets - Leasing Arrangement	381	417
Deferred Tax Assets, Leasehold Maintenance	(73)	$ 0
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss	644
Income Taxes Receivable	225
State [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss	562
Operating Loss, Available to Offset Prior Year Income	$ 85