LONG-TERM DEBT - LINE OF CREDIT (Details) $ in Millions	3 Months Ended				9 Months Ended
	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) airline	Sep. 30, 2020 USD ($)
Line of Credit Facility [Line Items]
Proceeds from Issuance of Secured Debt					$ 363	$ 2,581
Adjustments to Additional Paid in Capital, Warrant Issued	$ 8	$ 8	$ 9	$ 7
Line of Credit Facility, Number of Credit Facilities | airline					3
Line of Credit Facility, Maximum Borrowing Capacity					$ 486
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					150
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					86
US Treasury CARES Act Loan [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity					$ 1,900
Line of Credit Facility, Maximum Month-end Outstanding Amount		$ 135