Earnings per share - Schedule of Earnings Per Share, Basic and Diluted (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				6 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2021	Mar. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net (loss)/earnings from continuing operations			$ 13.7		$ 56.9	$ 59.5
Mandatory preferred stock dividends	$ (4.1)		(4.1)		(8.1)	(8.1)
Net (loss)/earnings from continuing operations attributable to common shareholders	(14.3)		9.6		48.8	51.4
Net loss from discontinued operations			(131.4)	$ 0.3	0.0	(131.1)
Net (loss)/earnings attributable to common shareholders	$ (14.3)		$ (121.8)		$ 48.8	$ (79.7)
Basic average shares outstanding (shares)	68.4		69.1		68.5	69.1
Basic net (loss)/earnings per common share- continuing operations (in dollars per share)	$ (0.21)		$ 0.14		$ 0.71	$ 0.74
Basic net loss per common share- discontinued operations (in dollars per share)	0		(1.90)		0	(1.89)
Basic net (loss)/earnings per common share (in dollars per share)	$ (0.21)		$ (1.76)		$ 0.71	$ (1.15)
Weighted average shares of common stock - Diluted (shares)	68.4		69.5		68.8	69.8
Diluted net (loss)/earnings per common share- continuing operations (in dollars per share)	$ (0.21)		$ 0.14		$ 0.71	$ 0.74
Diluted net loss per common share- discontinued operations (in dollars per share)	0		(1.89)		0	(1.88)
Diluted net (loss)/earnings per common share (in dollars per share)	$ (0.21)		$ (1.75)		$ 0.71	$ (1.14)
Restricted stock equivalents that were anti-dilutive	$ 0.1	$ 0.0	$ 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			1.3		1.3	1.1
Restricted Stock Equivalents
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			0.1
Deferred Compensation, Share-based Payments
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Effect of dilutive performance shares (shares)	0.0		0.1		0.1	0.1
Performance Shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Effect of dilutive performance shares (shares)	0.0		0.1		0.1	0.4
Restricted Stock Equivalents
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Effect of dilutive performance shares (shares)	0.0		0.2		0.1	0.2