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Property, plant and equipment (Narrative) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Mar. 02, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2018
Jul. 01, 2018
Apr. 01, 2018
Jan. 01, 2018
Dec. 01, 2017
Disclosure of detailed information about property, plant and equipment [line items]                
Borrowing costs capitalized as property, plant and equipment   $ 700          
Net book value transferred from non-producing properties to mining interests, net of pre-commercial production revenue   31,600            
Pre-commercial production revenue   4,000            
Impairment of property, plant and equipment   204          
Historical carrying value   $ 25,200            
Santacruz Silver Mining Ltd [Member]                
Disclosure of detailed information about property, plant and equipment [line items]                
Cash consideration $ 15,000              
Initial option payment 5,000              
Hochschild [Member]                
Disclosure of detailed information about property, plant and equipment [line items]                
Initial option payment 2,000              
Final option payment $ 8,000             $ 8,000
Hochschild [Member] | Events After Reporting Period [Member]                
Disclosure of detailed information about property, plant and equipment [line items]                
Final option payment       $ 6,000 $ 1,000 $ 500 $ 500