Retirement Benefits (Other Postretirement Benefit Plan Accumulated Benefit Obligation) (Details) - Other Postretirement Benefits Plan - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Obligation at beginning of year	$ 86.3	$ 86.9
Service cost	1.1	2.2	$ 2.8
Interest cost	1.6	2.0	2.2
Actuarial (gain) loss	(7.2)	7.5
Curtailment gain	(3.5)	0.0
Benefits paid	(11.3)	(12.3)
Obligation at end of year	$ 67.0	$ 86.3	$ 86.9