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Income and other taxes - Income tax expense (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Current tax recovery (expense):    
Current period before undernoted items $ (38,953) $ (117,496)
Benefit from unrecognised loss carry forwards 0 23,860
Adjustments to prior years 144 2,609
Current tax recovery (expense) (38,809) (91,027)
Deferred tax recovery (expense):    
Origination and reversal of temporary differences 31,389 (56,258)
Adjustments to prior years (138) (2,331)
Changes in tax rates 2,141 35
Other 943 (3,910)
Deferred tax recovery (expense) 34,335 (62,464)
Tax (expense) $ (4,474) $ (153,491)