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Shareholders' equity - Dividends (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Shareholders' equity    
Dividends declared $ 167.9 $ 156.8
Dividend declared per share $ 0.91 $ 0.87
Cash dividends declared $ 125.8 $ 118.1
Share dividends paid - DRIP $ 42.1 $ 38.7