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, 2020	Dec. 31, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net earnings from continuing operations	$ 67.1	$ 45.8
Mandatory preferred stock dividends	(4.0)	(4.0)
Net earnings from continuing operations attributable to common shareholders	63.1	41.8
Net earnings from discontinued operations	0.0	0.3
Net earnings attributable to common shareholders	$ 63.1	$ 42.1
Basic average shares outstanding (shares)	68.5	69.1
Basic net (loss)/earnings per common share- continuing operations (in dollars per share)	$ 0.92	$ 0.60
Basic net loss per common share- discontinued operations (in dollars per share)	0	0.01
Basic net (loss)/earnings per common share (in dollars per share)	$ 0.92	$ 0.61
Weighted average shares of common stock - Diluted (shares)	73.5	70.2
Diluted net (loss)/earnings per common share- continuing operations (in dollars per share)	$ 0.91	$ 0.60
Diluted net loss per common share- discontinued operations (in dollars per share)	0	0
Diluted net (loss)/earnings per common share (in dollars per share)	$ 0.91	$ 0.60
Restricted stock equivalents that were anti-dilutive	$ 0.0
Performance Shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	1.5	0.9
Restricted Stock Equivalents
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0.5	0.1
Deferred Compensation, Share-based Payments
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Effect of dilutive performance shares (shares)	0.1	0.2
Performance Shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Effect of dilutive performance shares (shares)	0.1	0.7
Restricted Stock Equivalents
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Effect of dilutive performance shares (shares)	0.1	0.2
MCPS
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Effect of dilutive performance shares (shares)	4.7	0.0