Stock Compensation Plans: Fair Value Assumptions (Details) - Stock Options - $ / shares	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility (as a percent)	33.00%	33.00%	35.00%
Expected life (in years)	5 years	5 years	5 years
Expected dividend yield (as a percent)	2.99%	3.61%	3.39%
Risk free interest rate (as a percent)	3.58%	4.17%	1.02%
Grant date fair value per share (in dollars per share)	$ 8.65	$ 6.87	$ 6.20