LONG-TERM DEBT - LINE OF CREDIT (Details)	3 Months Ended	6 Months Ended
	Sep. 30, 2018	Jun. 30, 2018 USD ($) credit_facility	Jul. 01, 2018 USD ($)
Line of Credit Facility [Line Items]
Line of Credit Facility, Number of Credit Facilities | credit_facility		3
Line of Credit Facility, Maximum Borrowing Capacity		$ 475,000,000
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		$ 75,000,000
Line of Credit Facility, Expiration Date		Sep. 30, 2018
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
Subsequent Event [Member] | 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