LONG-TERM DEBT - LINE OF CREDIT (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($) airline	Mar. 31, 2020 USD ($)
Line of Credit Facility [Line Items]
Proceeds from Issuance of Secured Debt	$ 189	$ 825
Adjustments to Additional Paid in Capital, Warrant Issued	$ 8
Line of Credit Facility, Number of Credit Facilities | airline	3
Line of Credit Facility, Maximum Borrowing Capacity	$ 461
Line of Credit Facility, Asset Restrictions	500
Credit Facility 1 [Member] | Secured by certain accounts receivable, spare engines, spare parts and ground service equipment [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	120
Credit Facility 2 [Member] | Secured by aircraft [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	250
Credit Facility 3 [Member] | Secured by aircraft [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	91
Credit Facilities 1 & 2 [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Month-end Outstanding Amount	363
US Treasury CARES Act Loan [Member]
Line of Credit Facility [Line Items]
Adjustments to Additional Paid in Capital, Warrant Issued	6
Line of Credit Facility, Maximum Borrowing Capacity	1,900
Line of Credit Facility, Maximum Month-end Outstanding Amount	$ 135