Earnings per share - Schedule of Earnings Per Share, Basic and Diluted (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended					9 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net earnings from continuing operations				$ 9.2		$ 88.5	$ 17.7
Mandatory preferred stock dividends	$ (4.0)			(4.4)		(12.1)	(7.7)
Net earnings from continuing operations attributable to common shareholders	25.0			4.8		76.4	10.0
Net loss from discontinued operations	0.8	$ (131.4)	$ 0.3	(1.8)	$ (11.0)	(130.3)	(12.8)
Net earnings/(loss) attributable to common shareholders	$ 25.8			$ 3.0		$ (53.9)	$ (2.8)
Basic average shares outstanding (shares)	68.5			69.6		68.9	65.5
Basic net (loss)/earnings per common share- continuing operations (in dollars per share)	$ 0.37			$ 0.07		$ 1.11	$ 0.15
Basic net loss per common share- discontinued operations (in dollars per share)	0.01			(0.03)		(1.89)	(0.19)
Basic net (loss)/earnings per common share (in dollars per share)	$ 0.38			$ 0.04		$ (0.78)	$ (0.04)
Weighted average shares of common stock - Diluted (shares)	68.7			70.6		69.4	66.5
Diluted net (loss)/earnings per common share- continuing operations (in dollars per share)	$ 0.37			$ 0.07		$ 1.10	$ 0.15
Diluted net loss per common share- discontinued operations (in dollars per share)	0.01			(0.03)		(1.88)	(0.19)
Diluted net (loss)/earnings per common share (in dollars per share)	$ 0.38			$ 0.04		$ (0.78)	$ (0.04)
Restricted stock equivalents that were anti-dilutive	$ 0.5	$ 0.0		$ 0.1		$ 0.3	$ 0.1
Performance Shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	1.4					1.2	0.8
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		0.1	0.2
Performance Shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Effect of dilutive performance shares (shares)	0.0			0.5		0.3	0.5
Restricted Stock Equivalents
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Effect of dilutive performance shares (shares)	0.1			0.3		0.1	0.3