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, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net earnings	$ 95.6	$ 65.5
Basic average shares outstanding (shares)	61.8	62.0
Effect of dilutive restricted stock equivalents (shares)	0.6	0.3
Effect of dilutive performance shares (shares)	0.5	0.0
Diluted average shares outstanding (shares)	62.9	62.3
Basic earnings per common share (dollars per share)	$ 1.55	$ 1.06
Diluted earnings per common share (dollars per share)	$ 1.52	$ 1.05
Securities not included in calculation (shares)		0.0