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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
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      
Shareholders' equity      
Number of shares issued at At-the-market equity offering     1,054,800