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Financial instruments and risk management - Fuel Sensitivity (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Financial instruments and risk management    
+ US$0.01 per gallon $ 0.01 $ 0.01
- US$0.01 per gallon $ 0.01 $ 0.01
Increase in Operating costs $ 2,731 $ 1,762
Decrease in Operating costs $ (2,731) $ (1,762)