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Maintenance rights intangible and lease premium, net (Estimated future amortization expense for lease premium) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]    
2018 $ 24,870  
2019 21,176  
2020 21,176  
2021 21,176  
2022 21,176  
Thereafter 177,238  
Finite-lived intangible assets 286,812 $ 317,988
Lease Premiums [Member]    
Finite-Lived Intangible Assets [Line Items]    
2018 11,219  
2019 10,466  
2020 7,727  
2021 5,394  
2022 914  
Thereafter 1,539  
Finite-lived intangible assets $ 37,259 $ 50,891