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Financial instruments and risk management - Fuel Sensitivity (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Financial instruments and risk management      
+ US$0.01 per gallon $ 0.01 $ 0.01 $ 0.01
- US$0.01 per gallon $ 0.01    
Increase in operating costs $ 3,719 $ 3,399 $ 2,731
Decrease in operating costs $ (3,719) $ (3,399) $ (2,731)