Retirement Benefits - Schedule of Accumulated and Project Benefit Obligations (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan, Pension Plan with Project Benefit Obligation in Excess of Plan Assets [Abstract]
Projected benefit obligation	$ 4,607.4	$ 3,755.5
Fair value of plan assets	3,446.7	3,220.2
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Accumulated benefit obligation	4,019.7	679.7
Fair value of plan assets	$ 3,205.2	$ 392.9