Earnings per share (Schedule of Earnings Per Share, Basic and Diluted) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net earnings from continuing operations									$ 64.7	$ 93.5	$ 201.5
Mandatory preferred stock dividends									(12.0)	0.0	0.0
Net earnings from continuing operations attributable to common shareholders									52.7	93.5	201.5
Loss from discontinued operations, net of tax									(13.6)	0.0	0.0
Net earnings attributable to common shareholders									$ 39.1	$ 93.5	$ 201.5
Basic average shares outstanding (in shares)									66.4	59.8	61.7
Basic net earnings per common share - continuing operations (in dollars per share)									$ 0.79	$ 1.56	$ 3.27
Basic net loss per common share - discontinued operations (in dollars per share)									(0.20)	0	0
Basic net earnings per common share (in dollars per share)	$ 0.62	$ 0.07	$ (0.97)	$ 1.19	$ 0.03	$ 0.40	$ 0.13	$ 1.00	$ 0.59	$ 1.56	$ 3.27
Effect of dilutive restricted stock equivalents (in shares)									0.3	0.5	0.5
Effect of dilutive performance shares (in shares)									0.2	0.2	0.0
Diluted average shares outstanding (in shares)									67.3	61.4	62.6
Diluted net earnings per common share - continuing operations (in dollars per share)									$ 0.78	$ 1.52	$ 3.22
Diluted net loss per common share - discontinued operations (in dollars per share)									(0.20)	0	0
Diluted net earnings per common share (in dollars per share)	$ 0.62	$ 0.07	$ (0.97)	$ 1.16	$ 0.02	$ 0.39	$ 0.13	$ 0.98	$ 0.58	$ 1.52	$ 3.22
Performance Shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Effect of dilutive performance shares (in shares)									0.4	0.9	0.4