GENERAL AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - MILEAGE PLAN (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Mileage Plan [Line Items]
Other accrued liabilities	$ 918	$ 878
Deferred revenue	699	640
Other liabilities	451	417
Deferred revenue from Mileage Plan awards issued but not yet flown	47	43
Passenger revenues	6,818	5,006	$ 4,793
Other-net revenues	1,001	817	697
Total Operating Revenues	7,933	5,931	5,598
Revenue From Mileage Plan [Member]
Mileage Plan [Line Items]
Passenger revenues	380	293	267
Other-net revenues	482	429	329
Total Operating Revenues	862	722	596
Commission revenue	396	329	$ 280
Liabilities From Mileage Plan [Member]
Mileage Plan [Line Items]
Other accrued liabilities	519	484
Deferred revenue	699	638
Other liabilities	26	21
Liabilities	$ 1,244	$ 1,143