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Earnings per share - Schedule of Earnings Per Share, Basic and Diluted (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
3 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Earnings Per Share [Abstract]    
Net earnings $ 60.4 $ 95.6
Basic average shares outstanding (shares) 60.2 61.8
Effect of dilutive restricted stock equivalents (shares) 0.4 0.6
Effect of dilutive performance shares (shares) 0.9 0.5
Diluted average shares outstanding (shares) 61.5 62.9
Basic earnings per common share (dollars per share) $ 1.00 $ 1.55
Diluted earnings per common share (dollars per share) $ 0.98 $ 1.52