INCOME TAXES - DEFERRED TAX ASSETS AND LIABILITIES (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Excess of tax over book depreciation	$ 1,066	$ 964
Deferred Tax Liabilities, Intangible Assets	15	14
Deferred Tax Liabilities, Other	43	88
Gross deferred tax liabilities	1,124	1,066
Mileage Plan	(315)	(337)
Inventory obsolescence	(15)	(16)
Deferred gains	(5)	(5)
Employee benefits	(172)	(154)
Acquired net operating losses	(64)	(127)
Other—net	(43)	(57)
Deferred tax assets	(614)	(696)
Valuation allowance	2	0
Net deferred tax liabilities	512	$ 370
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss	243
State [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss	$ 189
Minimum [Member] | Federal [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2032
Minimum [Member] | State [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2029
Maximum [Member] | Federal [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2036
Maximum [Member] | State [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2036