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Income and other taxes - Income tax expense (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Current tax recovery (expense):    
Current period before undernoted items $ (27,759) $ (38,953)
Adjustments to prior years 2,563 144
Current tax recovery (expense) (25,196) (38,809)
Deferred tax recovery (expense):    
Origination and reversal of temporary differences 89,301 31,389
Adjustments to prior years (1,067) (138)
Changes in tax rates (5,031) 2,141
Other 4,098 943
Deferred tax recovery (expense) 87,301 34,335
Tax (expense) recovery $ 62,105 $ (4,474)