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Financial risk management (Schedule of Fair Value Measurements) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Disclosure of fair value measurement of assets [line items]        
Cash and cash equivalents $ 3,111 $ 9,325 $ 8,723 $ 24,055
Restricted cash 709 335    
Trade and other receivables 5,646 6,631    
Forward contracts 263    
Pre-payment facility 12,230 15,000    
Level 1 [Member]        
Disclosure of fair value measurement of assets [line items]        
Cash and cash equivalents 3,111 9,325    
Restricted cash 709 335    
Level 2 [Member]        
Disclosure of fair value measurement of assets [line items]        
Trade and other receivables 5,646 6,631    
Forward contracts 263    
Pre-payment facility $ 12,230 $ 15,000