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Shareholders' equity - Dividends (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Shareholders' equity    
Dividend declared per share $ 0.99 $ 0.95
Cash dividends $ 138.2 $ 136.1
DRIP dividends 48.8 41.7
Dividends declared $ 187.0 $ 177.8