Condensed Financial Information - Parent Company Only: Condensed Statements of Operations, Parent Company (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Operating income
Interest on deposits in banks									$ 1,117	$ 2,535	$ 5,172
Interest on investment securities									1,195	1,579	1,264
Income before income taxes and equity in undistributed     income of Bank	$ 7,543	$ 8,810	$ 8,902	$ 9,173	$ 7,992	$ 7,674	$ 6,274	$ 8,367	34,428	30,307	29,921
Benefit for income taxes	1,525	1,786	1,651	1,883	1,635	1,463	1,225	1,715	6,845	6,038	5,901
Net income	$ 6,018	$ 7,024	$ 7,251	$ 7,290	$ 6,357	$ 6,211	$ 5,049	$ 6,652	27,583	24,269	24,020
Parent Company
Operating income
Interest on deposits in banks									5	26	67
Interest on loan receivable from ESOP									0	0	9
Interest on investment securities									24	5	0
Dividends from Bank									9,085	8,762	6,607
Total operating income									9,114	8,793	6,683
Operating expenses									495	554	525
Income before income taxes and equity in undistributed     income of Bank									8,619	8,239	6,158
Benefit for income taxes									(238)	(186)	(169)
Income before undistributed income of Bank									8,857	8,425	6,327
Equity in undistributed income of Bank									18,726	15,844	17,693
Net income									$ 27,583	$ 24,269	$ 24,020