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Stock-based Compensation Assumptions Used in Black-Scholes Option Valuation Model (Details) - Employee Stock Option
12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Stock Based Compensation Activity [Line Items]      
Expected life of option (years) 6 years 183 days   6 years 146 days
Risk-free interest rate, minimum 2.80% 1.23% 1.64%
Risk-free interest rate, maximum 2.80% 1.93% 1.70%
Expected volatility of the Company's stock 23.70% 24.60% 36.00%
Expected dividend yield on the Company's stock 2.78% 2.21% 2.11%
Minimum      
Stock Based Compensation Activity [Line Items]      
Expected life of option (years) 4 years 272 days  
Maximum      
Stock Based Compensation Activity [Line Items]      
Expected life of option (years) 6 years 182 days