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, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net (loss)/earnings from continuing operations	$ (62.3)	$ 7.8	$ 8.5	$ 68.2
Mandatory preferred stock dividends	(3.3)	0.0	(3.3)	0.0
Net (loss)/earnings from continuing operations attributable to common shareholders	(65.6)	7.8	5.2	68.2
Loss from discontinued operations, net of tax	(11.0)	0.0	(11.0)	0.0
Net (loss)/earnings attributable to common shareholders	$ (76.6)	$ 7.8	$ (5.8)	$ 68.2
Basic average shares outstanding (shares)	67.3	59.7	63.5	60.0
Basic net (loss)/earnings per common share- continuing operations (in dollars per share)	$ (0.97)	$ 0.13	$ 0.08	$ 1.14
Basic net loss per common share- discontinued operations (in dollars per share)	(0.17)	0	(0.17)	0
Basic net (loss)/earnings per common share (in dollars per share)	$ (1.14)	$ 0.13	$ (0.09)	$ 1.14
Weighted average shares of common stock - Diluted (shares)	67.3	61.1	64.6	61.3
Diluted net (loss)/earnings per common share- continuing operations (in dollars per share)	$ (0.97)	$ 0.13	$ 0.08	$ 1.11
Diluted net loss per common share- discontinued operations (in dollars per share)	(0.17)	0	(0.17)	0
Diluted net (loss)/earnings per common share (in dollars per share)	$ (1.14)	$ 0.13	$ (0.09)	$ 1.11
Amount reclassified from liability based plan to equity compensation plan	$ 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	0.0	0.2	0.0
Performance Shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Effect of dilutive performance shares (shares)	0.0	0.0	0.6	0.0
Restricted Stock Equivalents
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Effect of dilutive performance shares (shares)	0.0	1.4	0.3	1.3