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Leases - Additional Information Related to Leases (Details) - aircraft
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Sep. 30, 2018
Lessee, Lease, Description [Line Items]      
Weighted-average remaining lease term - finance leases 5 years 5 years  
Weighted-average remaining lease term - operating leases 10 years 10 years  
Weighted-average discount rate - finance leases 50.50% 6.70%  
Weighted-average discount rate - operating leases 5.20% 5.00%  
Embraer ERJ 145      
Lessee, Lease, Description [Line Items]      
Number of leased aircraft     54