DERIVATIVE INSTRUMENTS - FAIR VALUE OF HEDGE POSITIONS (Details) gallons in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) gallons aircraft numberOfInterestRateSwaps	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Payments for (Proceeds from) Hedge, Investing Activities	$ 19	$ (21)	$ 12
Aircraft Rent [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Price Risk Cash Flow Hedge Unrealized Gain (Loss) to be Reclassified During Next 12 Months	3
Interest Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Price Risk Cash Flow Hedge Unrealized Gain (Loss) to be Reclassified During Next 12 Months	$ 4
B737-800 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Capital Leased Assets, Number of Units | aircraft	6
Fuel hedge contracts [Member] | Derivative Instruments Not Designated as Hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount | gallons	435
Interest rate swaps agreements [Member] | Secured Debt [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount	$ 717
Number of interest rate swap agreements | numberOfInterestRateSwaps	13