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Shareholders' equity - Dividends (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Shareholders' equity    
Dividend declared per share $ 1.03 $ 0.99
Cash dividends $ (154.9) $ (138.2)
DRIP dividends (42.3) (48.8)
Dividends declared $ (197.2) $ (187.0)