COMMITMENTS AND CONTINGENCIES - Schedule of Purchase Commitment and Commitment Expenditures (Details) $ in Thousands	Nov. 08, 2017 USD ($) aircraft	Sep. 30, 2017 USD ($)
Aircraft | Subsequent Event
Number of Aircraft Unconditional Commitments
Six Months Ending March 31, 2018	0
2019	1
2020	4
2021	4
2022 and thereafter	18
Purchase Commitment - Total Aircrafts	27
Commitment Expenditures
Six Months Ending March 31, 2018 | $	$ 3,192
2019 | $	19,820
2020 | $	87,134
2021 | $	77,507
2022 and thereafter | $	280,876
Purchase Commitments - Total | $	$ 468,529
Number of Aircraft Conditional Commitments
Six Months Ending March 31, 2018	0
2019	2
2020	2
2021	0
2022 and thereafter	0
Aircraft Purchase Options - Total Aircrafts	4
Related Option Expenditures
Six Months Ending March 31, 2018 | $	$ 0
2019 | $	44,181
2020 | $	31,536
2021 | $	0
2022 and thereafter | $	0
Aircraft Purchase Options - Total | $	75,717
Unrecorded Unconditional Purchase Obligation Due Within Two and Three Years, Deferred Payment | $	130,000
Unrecorded Unconditional Purchase Obligation Deferral Due in Remainder of Fiscal Year | $	$ 62,000
Cancellable Commitments
Related Option Expenditures
Deposit assets | $		$ 4,500
Cancellable Commitments | Subsequent Event
Number of Aircraft Unconditional Commitments
Purchase Commitment - Total Aircrafts	5
Commitment Expenditures
Purchase Commitments - Total | $	$ 86,000
Large | Aircraft | Subsequent Event
Number of Aircraft Unconditional Commitments
Six Months Ending March 31, 2018	0
2019	1
2020	0
2021	4
2022 and thereafter	18
Purchase Commitment - Total Aircrafts	23
Number of Aircraft Conditional Commitments
Six Months Ending March 31, 2018	0
2019	2
2020	2
2021	0
2022 and thereafter	0
Aircraft Purchase Options - Total Aircrafts	4
U.K. SAR | Aircraft | Subsequent Event
Number of Aircraft Unconditional Commitments
Six Months Ending March 31, 2018	0
2019	0
2020	4
2021	0
2022 and thereafter	0
Purchase Commitment - Total Aircrafts	4
Commitment Expenditures
Six Months Ending March 31, 2018 | $	$ 3,192
2019 | $	0
2020 | $	61,994
2021 | $	0
2022 and thereafter | $	0
Purchase Commitments - Total | $	65,186
Medium and large | Aircraft | Subsequent Event
Commitment Expenditures
Six Months Ending March 31, 2018 | $	0
2019 | $	19,820
2020 | $	25,140
2021 | $	77,507
2022 and thereafter | $	280,876
Purchase Commitments - Total | $	$ 403,343