Commitments and Contingent Liabilities - Environmental Matters and Conditional Asset Retirement Obligations (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Accrual for Environmental Loss Contingencies [Abstract]
Other current liabilities	$ 7.8
Other liabilities	53.8
Total recorded environmental remediation costs	61.6	$ 67.6
Receivables	(1.4)
Other assets	(8.0)
Total recorded probable expected recoveries	(9.4)
Net environmental remediation costs	52.2
Discounted ongoing operating maintenance expenditures	45.7
Asset Retirement Obligation [Abstract]
Other current liabilities	0.6	0.7
Other liabilities	21.3	20.5
Total recorded conditional asset retirement obligations	21.9	21.2
Receivables	0.0	(0.1)
Other assets	(0.3)	(0.2)
Total recorded probable expected recoveries	(0.3)	(0.3)
Net conditional asset retirement obligations	$ 21.6	$ 20.9