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Stock Compensation Plans: Fair Value Assumptions (Details) - Stock Options - $ / shares
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 (as a percent) 17.00% 16.00% 16.00%
Expected life (in years) 5 years 5 years 5 years
Expected dividend yield (as a percent) 2.61% 1.85% 3.00%
Risk free interest rate (as a percent) 2.97% 1.89% 1.12%
Grant date fair value per share (in dollars per share) $ 4.476 $ 3.84 $ 1.46