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Financial risk management (Narrative) (Details)
 3 Months Ended 9 Months Ended
Sep. 30, 2018
USD ($)
lb
Sep. 30, 2018
USD ($)
lb
Dec. 31, 2017
USD ($)
Non-Hedge Foreign Exchange Forward Contracts [Member] | Zinc [Member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Gain from sell of commodity $ 100,000 $ 200,000  
Quantity of commodity sell | lb 700,000 1,400,000  
Non-Hedge Foreign Exchange Forward Contracts [Member] | Lead [Member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Gain from sell of commodity   $ 300,000  
Quantity of commodity sell | lb   2,600,000  
Credit risk [Member] | Trade and other receivables [Member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Risk exposure associated with instrument $ 3,700,000 $ 3,700,000 $ 3,800,000
Credit risk [Member] | Mexican Value Added Taxes [Member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Risk exposure associated with instrument 1,700,000 1,700,000 $ 2,800,000
Silver, Zinc, Lead and Copper Prices [Member] | Trade and other receivables [Member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Risk exposure associated with instrument $ 400,000 $ 400,000  
Interest rate risk [Member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Interest rate basis spread over U.S. LIBOR 5.00% 5.00%