LONG-TERM DEBT - LINE OF CREDIT (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) credit_facility	Sep. 30, 2020 USD ($)	Sep. 01, 2020 USD ($)	Jun. 30, 2020 USD ($)
Line of Credit Facility [Line Items]
Line of credit facility, borrowing capacity	$ 461,000,000
Number of credit facilities | credit_facility	3
Line of Credit Facility, Periodic Payment, Principal	$ 37,000,000
Proceeds from Lines of Credit	400,000,000
Credit Facility 1 [Member]
Line of Credit Facility [Line Items]
Line of credit facility, asset restrictions, unrestricted cash and marketable securities	$ 500,000,000
Credit Facility 1 [Member] | Secured by aircraft [Member]
Line of Credit Facility [Line Items]
Line of credit facility, borrowing capacity				$ 250,000,000
Number of credit facilities | credit_facility	1
Credit Facility 2 [Member] | Secured by other [Member]
Line of Credit Facility [Line Items]
Line of credit facility, borrowing capacity	$ 120,000,000		$ 150,000,000
Credit Facility 3 [Member] | Secured by aircraft [Member]
Line of Credit Facility [Line Items]
Line of credit facility, borrowing capacity		$ 91,000,000
US Treasury CARES Act Loan
Line of Credit Facility [Line Items]
Line of credit facility, borrowing capacity	1,900,000,000
Proceeds from Lines of Credit	$ 135,000,000