INCOME TAXES - DEFERRED TAX ASSETS AND LIABILITIES (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Excess of tax over book depreciation	$ 1,605	$ 1,312
Deferred Tax Liabilities, Intangible Assets	16	17
Deferred Tax Liabilities, Other	3	49
Gross deferred tax liabilities	1,916	1,734
Mileage Plan	(455)	(436)
Inventory obsolescence	(23)	(21)
Employee benefits	(159)	(145)
Net operating losses	(166)	(28)
Other - net	(77)	(88)
Deferred tax assets	(1,237)	(1,177)
Valuation allowance	16	17
Net deferred tax liabilities	695	574
Operating Loss Carryforwards [Line Items]
Valuation allowance	16	17
Deferred Tax Liabilities, Other Finite-Lived Assets	292	356
Deferred Tax Assets - Leasing Arrangement	329	392
Deferred Tax Assets, Leasehold Maintenance	(28)	$ (67)
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss	684
Income Taxes Receivable	4
State [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss	$ 467