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, 2018	Dec. 31, 2017
Reconciled Basic and Diluted EPS
Net income (loss)	$ 80.3	$ (236.4)
Less: Allocation to participating securities	(0.1)	0.2
Net income available to common shareowners	$ 80.2	$ (236.2)
Basic weighted average outstanding shares (in shares)	120.3	128.2
Effect of dilutive securities
Stock options (in shares)	1.0	0.0
Performance shares (in shares)	0.2	0.0
Diluted weighted average outstanding shares (in shares)	121.5	128.2
Earnings (loss) per share:
Basic (in usd per share)	$ 0.67	$ (1.84)
Diluted (in usd per share)	$ 0.66	$ (1.84)