Annual report pursuant to Section 13 and 15(d)

Retirement Benefits - Postretirement Healthcare Plan (Details)

v3.8.0.1
Retirement Benefits - Postretirement Healthcare Plan (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
One-percentage-point increase/decrease in the assumed healthcare cost trend rates      
Accrued pension costs, long-term portion $ 129.6 $ 232.7  
Other Postretirement Benefit Plan [Member]      
Retirement and Postemployment Benefits      
Other Postretirement Benefit Expense $ 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) 3.30% 3.10% 3.70%
Healthcare cost rate assumed for next year (as a percent) 7.60% 7.30% 7.60%
Ultimate trend rate (as a percent) 4.50% 4.50% 4.50%
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 0.6    
Impact of a one percentage point decrease in healthcare cost trends on the benefit obligation 0.6    
Employer's expected annual future contribution to the postretirement healthcare plan 0.8    
Defined Benefit Plan, Benefit Obligation 9.0 $ 10.3 $ 10.3
Interest cost 0.2 0.3  
Service cost 0.4 0.4  
Defined Benefit Plan, Actuarial Gain (Loss) (0.9) 0.2  
Defined Benefit Plan, Benefits Paid 1.0 0.9  
Accrued pension costs, current portion 0.8 0.9  
Accrued pension costs, long-term portion 8.2 9.4  
Net amount recognized $ 9.0 $ 10.3