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Other operating expenses (Details Textual) - USD ($)
 12 Months Ended 60 Months Ended 72 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2013
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Dec. 31, 2013
Dec. 31, 2011
Disclosure of other operating expenses [Abstract]                
Disputed royalties for december two thousand six through september two thousand eleven   $ 174,800,000            
Net asset tax relating to royalties   33,600,000 $ 12,100,000 $ 12,100,000 $ 12,100,000 $ 12,100,000    
Profit sharing adjustments related to mining royalties $ 3,600,000 29.2            
Penalties on disputed royalties   $ 6,200,000         $ 17,700,000 $ 33,800,000