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Significant accounting policies - Stock-based compensation (Details)
12 Months Ended
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Number of trading days used to determine weighted average trading price of common shares 30 days
Restricted Share Unit (RSU)  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Award service period 2 years
Award vesting period 3 years
Compensation expense recognition period 3 years
Phantom Share Units (PSUs)  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Award vesting period 3 years
Compensation expense recognition period 3 years