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Financial instruments (Details) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Recurring    
Net put/call option liability   $ (680,000)
Net asset arising from put/call options $ 280,000  
Fair Value, Inputs, Level 1 [Member]    
Recurring    
Net put/call option liability   0
Net asset arising from put/call options 0  
Fair Value, Inputs, Level 2 [Member]    
Recurring    
Net put/call option liability   0
Net asset arising from put/call options 0  
Fair Value, Inputs, Level 3 [Member]    
Recurring    
Net put/call option liability   $ (680,000)
Net asset arising from put/call options $ 280,000