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Share capital (Share Issuances) (Narrative) (Details) - USD ($)
 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Disclosure of classes of share capital [line items]        
Number of shares reserved for issuance as a percent of total common shares issued and oustanding 10.00%      
Maximum term of stock options Under the plan, the Board of Directors determines the term of a stock option to a maximum of 10 years      
Weighted average fair value at grant $ 1.5 $ 1.58    
Shares outstanding, amount $ 89,709,000 $ 87,447,000 $ 88,058,000 $ 87,144,000
Restricted Share [Member]        
Disclosure of classes of share capital [line items]        
Shares outstanding, share 86,692   208,722  
Shares outstanding, amount $ 200,000   $ 800,000  
Deferred Share Units [Member]        
Disclosure of classes of share capital [line items]        
Shares outstanding, share 314,029   286,920  
Deferred Share Units [Member] | Bottom of range [member]        
Disclosure of classes of share capital [line items]        
Percentage of quarterly director fee payments earned as awards of deferred share units 20.00%      
Deferred Share Units [Member] | Top of range [member]        
Disclosure of classes of share capital [line items]        
Percentage of quarterly director fee payments earned as awards of deferred share units 100.00%