Annual report pursuant to Section 13 and 15(d)

Retirement Benefits - Postretirement Healthcare Plan (Details)

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Retirement Benefits - Postretirement Healthcare Plan (Details) - Other Postretirement Benefit Plans - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2013
Retirement and Postemployment Benefits      
Postretirement healthcare benefit cost $ 0.3 $ 0.3 $ 0.6
Weighted average assumptions used in revaluing obligation under the postretirement healthcare plan      
Discount rate for obligation (as a percent) 3.70% 3.70% 3.50%
Healthcare cost rate assumed for next year (as a percent) 7.60% 7.20% 7.70%
Ultimate trend rate (as a percent) 4.50% 5.00% 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 $ 0.9