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Shareholders' equity - At-the-market equity offering (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
May 11, 2020
Jul. 19, 2019
Jun. 30, 2020
Jun. 30, 2020
Dec. 31, 2019
Shareholders' equity          
Maximum value of shares that can be issued from treasury to public 300,000,000 200,000,000      
Number of shares issued at At-the-market equity offering     474,900 909,900  
Average price per common share     $ 143.58 $ 125.54  
Gross proceeds from share issues     $ 68.2 $ 114.2  
Net proceeds from share issues     66.8 112.3  
Share issuance agent commission costs     0.7 1.1  
Share issue costs and expense     $ 0.7 $ 0.8  
Share Capital (Note 15)          
Shareholders' equity          
Number of shares issued at At-the-market equity offering       909,900 1,433,400