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Income and other taxes - Change in deferred income tax liabilities (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Net    
Net, beginning of period $ (153,639) $ (223,757)
Deferred income tax recovery (expense) included in net income (loss) (10,284) 63,956
Impact of U.S. tax rate change in other comprehensive income (8,621) 0
Deferred income tax recovery (expense) included in other comprehensive income (loss) 9,295 6,597
Other (842) (435)
Net, end of period (164,091) (153,639)
Deferred tax assets    
Deferred tax assets, beginning of period 137,341 61,881
Deferred income tax recovery (expense) included in net income (loss) (34,517) 69,110
Impact of U.S. tax rate change in other comprehensive income (8,621) 0
Deferred income tax recovery (expense) included in other comprehensive income (loss) 8,398 6,364
Other (260) (14)
Deferred tax assets, end of period 102,341 137,341
Deferred tax liabilities    
Deferred tax liabilities, beginning of period (290,980) (285,638)
Deferred income tax recovery (expense) included in net income (loss) 24,233 (5,154)
Impact of U.S. tax rate change in other comprehensive income 0 0
Deferred income tax recovery (expense) included in other comprehensive income (loss) 897 233
Other (582) (421)
Deferred tax liabilities, end of period $ (266,432) $ (290,980)