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Supplementary cash flow information - Changes in non-cash working capital (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Statement of cash flows [abstract]    
Accounts receivable and prepaids $ 3,345 $ (6,768)
Value-added and other tax receivables 4,272 (4,441)
Inventories (41,186) (58,059)
Accounts payable and accrued liabilities 1,361 20,386
Current income and other taxes payable 40,838 9,201
Changes in non-cash working capital $ 8,630 $ (39,681)