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Income and other taxes - Income tax expense (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Current tax recovery (expense):    
Current period before undernoted items $ (85,287) $ (44,743)
Impact of Argentina gas settlement 0 (7,800)
Adjustments to prior years (217) (2,134)
Current tax recovery (expense) (85,504) (54,677)
Deferred tax recovery (expense):    
Origination and reversal of temporary differences 23,310 82,838
Impact of Argentina gas settlement 0 (3,575)
Derecognition of non-capital loss carryforwards 0 (17,861)
Adjustments to prior years 200 1,667
Change in U.S. tax rate (36,567) 0
Change in other jurisdictions tax rates 734 0
Other 2,039 887
Deferred tax recovery (expense) (10,284) 63,956
Tax expense (income), continuing operations $ (95,788) $ 9,279