Leases and Prepaid Maintenance Deposits - Future Minimum Lease Payments Under Noncancelable Operating Leases (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Total Operating and Capital Lease Obligations
2018	$ 250,566
2019	226,462
2020	206,900
2021	183,411
2022	162,143
2023 and thereafter	481,119
Total minimum lease payments	1,510,601
Office Equipment
Capital Leases
2018	537
2019	504
2020	188
2021	28
2022	0
2023 and thereafter	0
Total minimum lease payments	1,257
Less amount representing interest	94
Present value of minimum lease payments	1,163
Less current portion	476
Long-term portion	687
Aircraft and Spare Engine Leases
Operating Leases
2018	204,292
2019	189,106
2020	180,842
2021	170,643
2022	150,212
2023 and thereafter	419,908
Total minimum lease payments	1,315,003
Property Facility Leases
Operating Leases
2018	45,737
2019	36,852
2020	25,870
2021	12,740
2022	11,931
2023 and thereafter	61,211
Total minimum lease payments	$ 194,341

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