Loans Receivable and Allowance for Loan Losses: Impaired Financing Receivables (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	$ 3,764	$ 4,282	$ 4,568
Impaired Financing Receivable, with Related Allowance, Recorded Investment	245	250	656
Impaired Financing Receivable, Recorded Investment	4,009	4,532	5,224
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance (Loan Balance Plus Charge Off)	3,857	4,375	4,615
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance (Loan Balance Plus Charge Off)	245	250	656
Impaired Financing Receivable, Unpaid Principal Balance (Loan Balance Plus Charge Off)	4,102	4,625	5,271
Impaired Financing Receivable, Related Allowance	123	127	247
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	4,102	4,567	4,750
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	247	413	454
Impaired Financing Receivable, Average Recorded Investment	4,349	4,980	5,204
Impaired Financing Receivable, Interest Income, Accrual Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	213	189	220
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	0	0	0
Impaired Financing Receivable, Interest Income Recognized	213	189	220
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Cash Basis Interest Income Recognized	172	159	182
Impaired Financing Receivable, with Related Allowance, Cash Basis Interest Income Recognized	0	0	0
Impaired Financing Receivable, Cash Basis Interest Income Recognized	172	159	182
Mortgage loans, one-to-four family
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	368	388	407
Impaired Financing Receivable, with Related Allowance, Recorded Investment			0
Impaired Financing Receivable, Recorded Investment	368	388	407
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance (Loan Balance Plus Charge Off)	412	432	450
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance (Loan Balance Plus Charge Off)			0
Impaired Financing Receivable, Unpaid Principal Balance (Loan Balance Plus Charge Off)	412	432	450
Impaired Financing Receivable, Related Allowance	0	0	0
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	378	470	655
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment			97
Impaired Financing Receivable, Average Recorded Investment	378	470	752
Impaired Financing Receivable, Interest Income, Accrual Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	29	31	58
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized			0
Impaired Financing Receivable, Interest Income Recognized	29	31	58
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Cash Basis Interest Income Recognized	29	31	52
Impaired Financing Receivable, with Related Allowance, Cash Basis Interest Income Recognized			0
Impaired Financing Receivable, Cash Basis Interest Income Recognized	29	31	52
Mortgage loans, commercial
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	2,973	2,988	3,143
Impaired Financing Receivable, Recorded Investment	2,973	2,988	3,143
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance (Loan Balance Plus Charge Off)	2,973	2,988	3,143
Impaired Financing Receivable, Unpaid Principal Balance (Loan Balance Plus Charge Off)	2,973	2,988	3,143
Impaired Financing Receivable, Related Allowance	0	0	0
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	2,987	3,041	3,039
Impaired Financing Receivable, Average Recorded Investment	2,987	3,041	3,039
Impaired Financing Receivable, Interest Income, Accrual Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	167	152	159
Impaired Financing Receivable, Interest Income Recognized	167	152	159
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Cash Basis Interest Income Recognized	129	123	127
Impaired Financing Receivable, Cash Basis Interest Income Recognized	129	123	127
Mortgage loans, land
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	0	450	321
Impaired Financing Receivable, with Related Allowance, Recorded Investment			362
Impaired Financing Receivable, Recorded Investment	0	450	683
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance (Loan Balance Plus Charge Off)	0	450	321
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance (Loan Balance Plus Charge Off)			362
Impaired Financing Receivable, Unpaid Principal Balance (Loan Balance Plus Charge Off)	0	450	683
Impaired Financing Receivable, Related Allowance	0	0	76
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	297	492	292
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment			72
Impaired Financing Receivable, Average Recorded Investment	297	492	364
Impaired Financing Receivable, Interest Income, Accrual Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	5	0	2
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized			0
Impaired Financing Receivable, Interest Income Recognized	5	0	2
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Cash Basis Interest Income Recognized	4	0	2
Impaired Financing Receivable, with Related Allowance, Cash Basis Interest Income Recognized			0
Impaired Financing Receivable, Cash Basis Interest Income Recognized	4	0	2
Consumer loans, home equity and second mortgage
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	382	394	516
Impaired Financing Receivable, with Related Allowance, Recorded Investment		0
Impaired Financing Receivable, Recorded Investment	382	394	516
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance (Loan Balance Plus Charge Off)	382	394	516
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance (Loan Balance Plus Charge Off)		0
Impaired Financing Receivable, Unpaid Principal Balance (Loan Balance Plus Charge Off)	382	394	516
Impaired Financing Receivable, Related Allowance	0	0	0
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	390	436	552
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		145
Impaired Financing Receivable, Average Recorded Investment	390	581	552
Impaired Financing Receivable, Interest Income, Accrual Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	12	6	1
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized		0
Impaired Financing Receivable, Interest Income Recognized	12	6	1
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Cash Basis Interest Income Recognized	10	5	1
Impaired Financing Receivable, with Related Allowance, Cash Basis Interest Income Recognized		0
Impaired Financing Receivable, Cash Basis Interest Income Recognized	10	5	1
Consumer loans, other
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment		3
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance (Loan Balance Plus Charge Off)		3
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		7
Impaired Financing Receivable, Interest Income, Accrual Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized		0
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Cash Basis Interest Income Recognized		0
Consumer loans, other
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	0
Impaired Financing Receivable, Recorded Investment	0	3
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance (Loan Balance Plus Charge Off)	0
Impaired Financing Receivable, Unpaid Principal Balance (Loan Balance Plus Charge Off)	0	3
Impaired Financing Receivable, Related Allowance	0	0
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	1
Impaired Financing Receivable, Average Recorded Investment	1	7
Impaired Financing Receivable, Interest Income, Accrual Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	0
Impaired Financing Receivable, Interest Income Recognized	0	0
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Cash Basis Interest Income Recognized	0
Impaired Financing Receivable, Cash Basis Interest Income Recognized	0	0
Consumer Loan, Other [Member]
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment			17
Impaired Financing Receivable, Recorded Investment			17
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance (Loan Balance Plus Charge Off)			17
Impaired Financing Receivable, Unpaid Principal Balance (Loan Balance Plus Charge Off)			17
Impaired Financing Receivable, Related Allowance			0
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment			12
Impaired Financing Receivable, Average Recorded Investment			12
Impaired Financing Receivable, Interest Income, Accrual Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized			0
Impaired Financing Receivable, Interest Income Recognized			0
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Cash Basis Interest Income Recognized			0
Impaired Financing Receivable, Cash Basis Interest Income Recognized			0
Commercial business loans
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	41	59	164
Impaired Financing Receivable, with Related Allowance, Recorded Investment	245	250	294
Impaired Financing Receivable, Recorded Investment	286	309	458
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance (Loan Balance Plus Charge Off)	90	108	168
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance (Loan Balance Plus Charge Off)	245	250	294
Impaired Financing Receivable, Unpaid Principal Balance (Loan Balance Plus Charge Off)	335	358	462
Impaired Financing Receivable, Related Allowance	123	127	171
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	49	121	200
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	247	268	285
Impaired Financing Receivable, Average Recorded Investment	296	389	485
Impaired Financing Receivable, Interest Income, Accrual Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	0	0	0
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	0	0	0
Impaired Financing Receivable, Interest Income Recognized	0	0	0
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Cash Basis Interest Income Recognized	0	0	0
Impaired Financing Receivable, with Related Allowance, Cash Basis Interest Income Recognized	0	0	0
Impaired Financing Receivable, Cash Basis Interest Income Recognized	$ 0	$ 0	$ 0