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Commitments and Contingencies (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
aircraft
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Future minimum rental payments required under operating leases      
2018 $ 142,157    
2019 104,297    
2020 121,119    
2021 112,657    
2022 75,981    
Thereafter 160,621    
Total future lease obligations $ 716,832    
Number of aircraft leased by the entity | aircraft 319    
Minimum rental expense for airport station rents $ 30,300 $ 31,400 $ 35,100