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Shareholders' equity - Common Shares (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Reconciliation Of Amount Of Shares Outstanding [Abstract]          
Balance at the beginning period     $ 5,580.1    
At-the-market equity offering       $ 112.3  
Amount of Exercise of stock options     0.3 6.0  
Dividend reinvestment plan $ 11.4 $ 10.1 19.5 $ 21.0  
Balance at the ending period $ 5,600.0   $ 5,600.0   $ 5,580.1
Common shares          
Shareholders' equity          
Shares issued 191,105,644   191,105,644    
Shares, par value (dollars per share) $ 0   $ 0    
Preferred shares          
Shareholders' equity          
Shares issued 0   0    
Share Capital          
Reconciliation of number of shares outstanding [abstract]          
Number of shares outstanding at beginning of period     190,956,476 189,381,388 189,381,388
At-the-market equity offering     0   1,054,800
Exercise of stock options     4,000   150,482
Vesting of restricted share units         42,328
Dividend reinvestment plan     145,168   327,478
Number of shares outstanding at end of period 191,105,644   191,105,644   190,956,476
Reconciliation Of Amount Of Shares Outstanding [Abstract]          
Balance at the beginning period     $ 5,580.1 $ 5,390.7 $ 5,390.7
At-the-market equity offering       112.3 133.7
Amount of Exercise of stock options     0.4 7.9 9.7
Amount of Vesting of restricted share units         3.7
Dividend reinvestment plan     19.5 $ 21.0 42.3
Balance at the ending period $ 5,600.0   $ 5,600.0   $ 5,580.1