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Accrued maintenance liability (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Movement in Accrued Maintenance Liability [Roll Forward]    
Accrued maintenance liability at beginning of period $ 2,461,799 $ 2,750,576
Maintenance payments received 743,256 756,314
Maintenance payments returned (459,326) (523,403)
Release to income upon sale 261,240 275,360
Release to income other than upon sale (228,081) (302,408)
Lessor contribution, top ups and other (27,581) 42,379
Interest accretion 8,667 13,701
Accrued maintenance liability at end of period $ 2,237,494 $ 2,461,799