INCOME TAXES - DEFERRED TAX ASSETS AND LIABILITIES (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Excess of tax over book depreciation	$ 1,282	$ 1,110
Deferred Tax Liabilities, Intangible Assets	39	0
Other—net	26	23
Gross deferred tax liabilities	1,347	1,133
Mileage Plan	(310)	(208)
Inventory obsolescence	(23)	(22)
Deferred gains	(8)	(8)
Employee benefits	(196)	(167)
Fuel hedge contracts	0	(5)
Acquired net operating losses	(289)	0
Other—net	(62)	(41)
Gross deferred tax assets	(888)	(451)
Valuation allowance	4	0
Net deferred tax liabilities	463	$ 682
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss	773
State [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss	$ 344