Accumulated Other Comprehensive Income (Loss) - Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Loss) Included in Computation of Net Periodic Pension and Postretirement Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Prior Service Costs
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	$ 0.7	$ 0.2	$ 0.9
Actuarial (Gains) Losses
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	3.1		2.0
Employee Pension and Postretirement Benefits, Net of Tax
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	3.8	0.2	2.9
Tax benefit	(1.0)	(0.1)	(0.7)
Net of tax	$ 2.8	$ 0.1	$ 2.2