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Share-Based Compensation (Market-Based Deferred Stock Unit Valuation Assumptions) (Details) - Market-Based Deferred Stock Unit
12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility 18.70% 20.60% 20.50%
Risk-free interest rate 1.90% 1.40% 1.20%
Expected dividend yield 2.60% 2.50% 2.20%