Retirement Benefits - Postretirement Healthcare Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
One-percentage-point increase/decrease in the assumed healthcare cost trend rates
Accrued pension costs, long-term portion	$ 232.7	$ 218.7
Other Postretirement Benefit Plans
Retirement and Postemployment Benefits
Postretirement healthcare benefit cost	$ 0.3	$ 0.3	$ 0.3
Weighted average assumptions used in revaluing obligation under the postretirement healthcare plan
Discount rate for obligation (as a percent)	3.10%	3.70%	3.70%
Healthcare cost rate assumed for next year (as a percent)	7.30%	7.60%	7.20%
Ultimate trend rate (as a percent)	4.50%	4.50%	5.00%
One-percentage-point increase/decrease in the assumed healthcare cost trend rates
Impact of a one percentage point increase in healthcare cost trends on service and interest costs	$ 0.1
Impact of a one percentage point decrease in healthcare cost trends on service and interest costs	0.1
Impact of a one percentage point increase in healthcare cost trends on the benefit obligation	0.7
Impact of a one percentage point decrease in healthcare cost trends on the benefit obligation	0.7
Employer's expected annual future contribution to the postretirement healthcare plan	0.8
Defined Benefit Plan, Benefit Obligation	10.3	$ 10.3	$ 8.9
Interest cost	0.3	0.3
Service cost	0.4	0.4
Defined Benefit Plan, Actuarial Gain (Loss)	0.2	1.1
Defined Benefit Plan, Benefits Paid	0.9	0.4
Accrued pension costs, current portion	0.9	0.9
Accrued pension costs, long-term portion	9.4	9.4
Net amount recognized	$ 10.3	$ 10.3