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Financial risk management (Narrative) (Details) | 3 Months Ended | 9 Months Ended |
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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% |

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