LONG-TERM DEBT - LINE OF CREDIT (Details)	9 Months Ended
	Sep. 30, 2019 USD ($) credit_facility	Sep. 30, 2018 USD ($)
Line of Credit Facility [Line Items]
Proceeds from Issuance of Secured Debt	$ 356,000,000	$ 0
Line of Credit Facility, Number of Credit Facilities | credit_facility	3
Line of Credit Facility, Maximum Borrowing Capacity	$ 516,000,000
Line of Credit Facility, Asset Restrictions	500,000,000
Credit Facility 1 [Member] | Secured by aircraft [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 250,000,000
Line of Credit Facility, Expiration Date	Jun. 30, 2021
Credit Facility 2 [Member] | Secured by aircraft [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 116,000,000
Line of Credit Facility, Expiration Date	Jul. 31, 2019
Credit Facility 3 [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	$ 150,000,000
Line of Credit Facility, Expiration Date	Mar. 31, 2022