COMMITMENTS AND CONTINGENCIES - Purchase Commitment Narrative (Details) $ in Thousands		3 Months Ended		9 Months Ended
	May 01, 2019 USD ($) aircraft	Dec. 31, 2018 USD ($) aircraft	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) aircraft	Dec. 31, 2017 USD ($)
Schedule Of Aircraft Purchase Contracts [Line Items]
Unrecorded Unconditional Purchase Obligation, Writedown Of Future Commitments | $		$ 17,500		$ 17,500
Contract termination costs | $		$ 13,971	$ 0	$ 13,971	$ 0
Aircraft
Schedule Of Aircraft Purchase Contracts [Line Items]
Purchase commitment period				7 years
Number of minimum quantity required in a purchase obligation		26		26
Number of minimum quantity required in purchase options		4		4
Unrecorded Unconditional Purchase Obligation, Minimum Quantity Required, Due In FY2022		5		5
2021		4		4
Cancellable Commitments
Schedule Of Aircraft Purchase Contracts [Line Items]
Number of minimum quantity required in a purchase obligation		5		5
H175 Aircraft [Member] | Aircraft Purchase Amendment [Member] | Subsequent Event [Member]
Schedule Of Aircraft Purchase Contracts [Line Items]
Number of minimum quantity required in a purchase obligation	22
Contract Postponement Period	18 months
Unrecorded Unconditional Purchase Obligation, Minimum Quantity Required, Due In FY2022	3
Capital expenditure deferral | $	$ 110,000
Unrecorded Unconditional Purchase Obligation, Due within Two and Three Years | $	14,000
Increase (Decrease) In Purchase Obligation | $	$ 18,400
H175 Aircraft [Member] | Aircraft Purchase Amendment [Member] | Subsequent Event [Member] | Cancellable Commitments
Schedule Of Aircraft Purchase Contracts [Line Items]
Number of minimum quantity required in a purchase obligation	5