COMMITMENTS AND CONTINGENCIES - Schedule of Purchase Commitment and Commitment Expenditures (Details) $ in Thousands	Sep. 30, 2018 USD ($) aircraft
Aircraft
Number of Aircraft Unconditional Commitments
Six Months Ending March 31, 2019	1
2020	4
2021	4
2022	5
2023 and thereafter	13
Purchase Commitment - Total Aircrafts	27
Commitment Expenditures
Six Months Ending March 31, 2019 | $	$ 19,780
2020 | $	85,608
2021 | $	76,149
2022 | $	84,530
2023 and thereafter | $	191,426
Purchase Commitments - Total | $	$ 457,493
Number of Aircraft Conditional Commitments
Six Months Ending March 31, 2019	2
2020	2
2021	0
2022	0
2023 and thereafter	0
Aircraft Purchase Options - Total Aircrafts	4
Related Option Expenditures
Six Months Ending March 31, 2019 | $	$ 44,181
2020 | $	31,536
2021 | $	0
2022 | $	0
2023 and thereafter | $	0
Aircraft Purchase Options - Total | $	$ 75,717
Cancellable Commitments
Number of Aircraft Unconditional Commitments
Purchase Commitment - Total Aircrafts	5
Commitment Expenditures
Purchase Commitments - Total | $	$ 92,600
Related Option Expenditures
Deposit assets | $	4,500
Large | Aircraft
Commitment Expenditures
Six Months Ending March 31, 2019 | $	19,780
2020 | $	24,700
2021 | $	76,149
2022 | $	84,530
2023 and thereafter | $	191,426
Purchase Commitments - Total | $	$ 396,585
Large | Aircraft
Number of Aircraft Unconditional Commitments
Six Months Ending March 31, 2019	1
2020	0
2021	4
2022	5
2023 and thereafter	13
Purchase Commitment - Total Aircrafts	23
Number of Aircraft Conditional Commitments
Six Months Ending March 31, 2019	2
2020	2
2021	0
2022	0
2023 and thereafter	0
Aircraft Purchase Options - Total Aircrafts	4
U.K. SAR | Aircraft
Number of Aircraft Unconditional Commitments
Six Months Ending March 31, 2019	0
2020	4
2021	0
2022	0
2023 and thereafter	0
Purchase Commitment - Total Aircrafts	4
Commitment Expenditures
Six Months Ending March 31, 2019 | $	$ 0
2020 | $	60,908
2021 | $	0
2022 | $	0
2023 and thereafter | $	0
Purchase Commitments - Total | $	$ 60,908