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Stock Compensation Plans: Fair Value Assumptions (Details) - Stock Options - $ / shares
12 Months Ended
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility (as a percent) 33.00% 35.00% 33.00%
Expected life (in years) 5 years 5 years 5 years
Expected dividend yield (as a percent) 3.61% 3.39% 5.36%
Risk free interest rate (as a percent) 4.17% 1.02% 0.28%
Grant date fair value per share (in dollars per share) $ 6.87 $ 6.20 $ 2.70