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Shareholders' equity - At-the-market equity offering (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
May 11, 2020
Apr. 28, 2020
Dec. 31, 2020
Dec. 31, 2019
Shareholders' equity        
Maximum value of shares that can be issued from treasury to public $ 300.0 $ 200.0    
Average price per common share     $ 128.96  
Gross proceeds from share issues     $ 136.0  
Net proceeds from share issues     133.7  
Share issuance agent commission costs     1.4  
Share issue costs and expense     $ 0.9  
Share Capital (Note 17)        
Shareholders' equity        
Number of shares issued at At-the-market equity offering     1,054,800 1,433,400