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Property, plant and equipment (Narrative) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Mar. 02, 2017
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2018
Jul. 02, 2018
Apr. 01, 2018
Jan. 02, 2018
Dec. 01, 2017
Disclosure of detailed information about property, plant and equipment [line items]                      
Borrowing costs capitalized as property, plant and equipment   $ 200 $ 500            
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          
Write-down of equipment   $ 65 $ 65 200          
Historical carrying value           25,200          
Insurance claim for equipment damaged from mining operations           $ 800          
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             $ 1,000 $ 500 $ 500 $ 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