INCOME TAXES - DEFERRED TAX ASSETS AND LIABILITIES (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Excess of tax over book depreciation	$ 964	$ 1,282
Deferred Tax Liabilities, Intangible Assets	14	39
Other—net	43	26
Gross deferred tax liabilities	1,021	1,347
Mileage Plan	(208)	(310)
Inventory obsolescence	(16)	(23)
Deferred gains	(5)	(8)
Employee benefits	(154)	(196)
Acquired net operating losses	(127)	(289)
Other—net	(57)	(62)
Gross deferred tax assets	(567)	(888)
Valuation allowance	0	4
Net deferred tax liabilities	454	$ 463
Operating Loss Carryforwards [Line Items]
Tax Cuts And Jobs Act Of 2017, Change In Tax Rate, Deferred Tax Asset, Income Tax Expense	280
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss	525
State [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss	$ 254
Minimum [Member] | Federal [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2029
Minimum [Member] | State [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2028
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, 2035