Basis of Presentation and Accounting Policies - Reconciliation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciled Basic and Diluted EPS
Net (loss) income	$ (236.4)	$ 214.7
Less: Allocation to participating securities	0.2	(0.2)
Net (loss) income available to common shareowners	$ (236.2)	$ 214.5
Basic weighted average outstanding shares	128.2	128.3
Effect of dilutive securities
Stock options	0.0	1.2
Performance shares	0.0	0.2
Diluted weighted average outstanding shares	128.2	129.7
(Loss) Earnings per share:
Basic	$ (1.84)	$ 1.67
Diluted	$ (1.84)	$ 1.65