Retirement Benefits - Schedule of Accumulated and Project Benefit Obligations (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan, Pension Plan with Project Benefit Obligation in Excess of Plan Assets [Abstract]
Projected benefit obligation	$ 4,482.0	$ 4,607.4
Fair value of plan assets	3,265.2	3,446.7
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Accumulated benefit obligation	4,113.4	4,019.7
Fair value of plan assets	$ 3,265.2	$ 3,205.2