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Share-based payment plans - Deferred Share Units (Details) - Deferred Share Units (DSU) - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Number of Awards    
Outstanding at the beginning of the period (in shares) 107,603  
Redeemed (in shares) (41,993) 0
Outstanding at the end of the period (in shares) 65,610 107,603
Weighted Average Issuance Price    
Outstanding at the beginning of the period (per share) $ 51.52  
Redeemed (per share) 49.96  
Outstanding at the end of the period (per share) $ 52.76 $ 51.52