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Retirement Benefits (Schedule of Health Care Cost Trend Rates) (Details) - Other Postretirement Benefit Plans [Member]
12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]    
Health care cost trend rate assumed for next year 6.50% 6.75%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) 5.00% 5.00%
Year that the rate reaches the ultimate trend rate 2025 2025