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Shareholders' equity - Common Shares (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Shareholders' equity    
Shares outstanding 186,692,481 185,930,331
Proceeds from exercise of stock options $ 4.2 $ 10.1
Proceeds from exercise of warrants   356.4
Common shares    
Shareholders' equity    
Shares issued 186,692,481  
Shares outstanding 186,692,481  
Shares, par value (dollars per share) $ 0  
Proceeds from exercise of stock options $ 4.2 $ 10.1
Exercise of stock options 94,018 433,718
Vesting of restricted share units 52,882 50,393
Common shares issued from dividend reinvestment plan 615,250 575,553
Preferred shares    
Shareholders' equity    
Shares issued 0