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, 2022	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2022	Mar. 31, 2021
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net earnings/(loss)		$ (10.2)		$ 79.0	$ 56.9
Mandatory preferred stock dividends	$ 0.0	(4.1)		(4.0)	(8.1)
Net earnings/(loss) attributable to common shareholders	$ 19.0	$ (14.3)		$ 75.0	$ 48.8
Basic average shares outstanding (shares)	70.4	68.4		68.6	68.5
Basic net (loss)/earnings per common share (in dollars per share)	$ 0.27	$ (0.21)		$ 1.09	$ 0.71
Weighted average shares of common stock - Diluted (shares)	71.6	68.4		69.0	68.8
Diluted net (loss)/earnings per common share (in dollars per share)	$ 0.27	$ (0.21)		$ 1.09	$ 0.71
Restricted stock equivalents that were anti-dilutive	$ 0.4		$ 0.0	$ 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	1.6			1.6	1.3
Deferred Compensation, Share-based Payments
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Effect of dilutive performance shares (shares)	0.1	0.0		0.1	0.1
Performance Shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Effect of dilutive performance shares (shares)	0.1	0.0		0.2	0.1
Restricted Stock Equivalents
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Effect of dilutive performance shares (shares)	0.1	0.0		0.1	0.1
MCPS
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Effect of dilutive performance shares (shares)	0.9	0.0		0.0	0.0