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Shareholders' equity - Dividends (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Shareholders' equity    
Dividends declared $ 177.8 $ 167.9
Dividend declared per share $ 0.95 $ 0.91
Cash dividends paid $ 136.1 $ 125.8
Share dividends paid - DRIP $ 41.7 $ 42.1