Retirement Benefits - Schedule of Amounts Included in Accumulated Other Comprehensive Loss, Net of Tax (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	$ 3.3	$ 2.7
Net actuarial loss	1,262.1	1,127.7
Total	1,265.4	1,130.4
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)		(4.1)
Net actuarial loss	5.8	7.4
Total	$ 5.8	$ 3.3