INCOME TAXES - DEFERRED TAX ASSETS AND LIABILITIES (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Excess of tax over book depreciation	$ 1,110,000,000	$ 1,042,000,000
Other—net	23,000,000	22,000,000
Gross deferred tax liabilities	1,133,000,000	1,064,000,000
Mileage Plan	(208,000,000)	(206,000,000)
Inventory obsolescence	(22,000,000)	(20,000,000)
Deferred gains	(8,000,000)	(10,000,000)
Employee benefits	(167,000,000)	(166,000,000)
Fuel hedge contracts	(5,000,000)	(5,000,000)
Other—net	(41,000,000)	(24,000,000)
Gross deferred tax assets	451,000,000	431,000,000
Net deferred tax liabilities	682,000,000	$ 633,000,000
Deferred tax assets valuation allowance	$ 0