INCOME TAXES - DEFERRED TAX ASSETS AND LIABILITIES (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Excess of tax over book depreciation	$ 1,233	$ 1,066
Deferred Tax Liabilities, Intangible Assets	16	15
Deferred Tax Liabilities, Other	58	43
Gross deferred tax liabilities	1,723	1,124
Mileage Plan	(337)	(315)
Inventory obsolescence	(15)	(15)
Employee benefits	(179)	(172)
Acquired net operating losses	(13)	(64)
Other - net	(48)	(48)
Deferred tax assets	(1,009)	(614)
Valuation allowance	1	2
Net deferred tax liabilities	715	512
Operating Loss Carryforwards [Line Items]
Deferred Tax Liabilities, Other Finite-Lived Assets	416	0
Deferred Tax Assets - Leasing Arrangement	417	$ 0
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss	45
State [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss	$ 55