DERIVATIVE INSTRUMENTS - FAIR VALUE OF HEDGE POSITIONS (Details) gallons in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) gallons aircraft agreement	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Derivative [Line Items]
Net cash received (paid) to for new positions and settlements	$ 21	$ (12)	$ (19)
B737-800 [Member]
Derivative [Line Items]
Capital Leased Assets, Number of B737-800 Aircraft | aircraft	6
Not Designated as Hedging Instrument [Member] | Fuel hedge contracts [Member]
Derivative [Line Items]
Fuel hedge contracts outstanding (in gallons) | gallons	421
Secured Debt [Member] | Interest rate swaps agreements [Member]
Derivative [Line Items]
Number of interest rate swap agreements | agreement	4
Derivative, Notional Amount	$ 382
Aircraft Rent [Member]
Derivative [Line Items]
Price Risk Cash Flow Hedge Unrealized Gain (Loss) to be Reclassified During Next 12 Months	(2)
Interest Income [Member]
Derivative [Line Items]
Price Risk Cash Flow Hedge Unrealized Gain (Loss) to be Reclassified During Next 12 Months	$ 3