Selected Quarterly Financial Data (Unaudited): Selected financial data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Quarterly Financial Information Disclosure [Abstract]
Interest and dividend income	$ 21,562	$ 19,889	$ 19,387	$ 19,112	$ 17,019	$ 14,627	$ 13,520	$ 13,342	$ 79,951	$ 58,508	$ 54,962
Interest expense	(4,731)	(3,255)	(2,236)	(1,369)	(756)	(645)	(627)	(646)	(11,592)	(2,674)	(3,104)
Net interest income	16,831	16,634	17,151	17,743	16,263	13,982	12,893	12,696	68,359	55,834	51,858
Provision for loan losses	(522)	(610)	(475)	(525)	(270)	0	0	0	(2,132)	(270)	0
Non-interest income	2,924	2,875	2,636	2,705	2,997	3,102	3,083	3,442	11,140	12,624	17,161
Non-interest expense	(10,967)	(10,927)	(10,944)	(10,535)	(10,155)	(9,874)	(9,333)	(9,264)	(43,373)	(38,626)	(34,591)
Income before income taxes	8,266	7,972	8,368	9,388	8,835	7,210	6,643	6,874	33,994	29,562	34,428
Provision for income taxes	1,624	1,666	1,705	1,881	1,785	1,472	1,316	1,389	6,876	5,962	6,845
Net income	$ 6,642	$ 6,306	$ 6,663	$ 7,507	$ 7,050	$ 5,738	$ 5,327	$ 5,485	$ 27,118	$ 23,600	$ 27,583
Net income per common share
Basic (in dollars per share)	$ 0.82	$ 0.77	$ 0.81	$ 0.91	$ 0.86	$ 0.69	$ 0.64	$ 0.66	$ 3.32	$ 2.84	$ 3.31
Diluted (in dollars per share)	$ 0.81	$ 0.77	$ 0.80	$ 0.90	$ 0.85	$ 0.69	$ 0.63	$ 0.65	$ 3.29	$ 2.82	$ 3.27