COMMITMENTS AND CONTINGENCIES - Schedule of Purchase Commitment and Commitment Expenditures (Details) $ in Thousands	Dec. 31, 2018 USD ($) aircraft
Aircraft
Number of Aircraft Unconditional Commitments
Three Months Ending March 31, 2019	0
2020	4
2021	4
2022	5
2023 and thereafter	13
Purchase Commitment - Total Aircrafts	26
Commitment Expenditures
Three Months Ending March 31, 2019 | $	$ 2,197
2020 | $	84,256
2021 | $	74,946
2022 | $	83,195
2023 and thereafter | $	188,401
Purchase Commitments - Total | $	$ 432,995
Number of Aircraft Conditional Commitments
Three 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
Three 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 | $	$ 91,100
Related Option Expenditures
Deposit assets | $	4,500
Large | Aircraft
Commitment Expenditures
Three Months Ending March 31, 2019 | $	2,197
2020 | $	24,310
2021 | $	74,946
2022 | $	83,195
2023 and thereafter | $	188,401
Purchase Commitments - Total | $	$ 373,049
Large | Aircraft
Number of Aircraft Unconditional Commitments
Three Months Ending March 31, 2019	0
2020	0
2021	4
2022	5
2023 and thereafter	13
Purchase Commitment - Total Aircrafts	22
Number of Aircraft Conditional Commitments
Three 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
Three Months Ending March 31, 2019	0
2020	4
2021	0
2022	0
2023 and thereafter	0
Purchase Commitment - Total Aircrafts	4
Commitment Expenditures
Three Months Ending March 31, 2019 | $	$ 0
2020 | $	59,946
2021 | $	0
2022 | $	0
2023 and thereafter | $	0
Purchase Commitments - Total | $	$ 59,946