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Financial risk management (Narrative) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
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 $ 3.8 $ 2.1
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 2.8 1.6
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 $ 0.4 $ 0.2