Loans and allowance for loan losses - Aging of recorded investment in past due loans and leases (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	$ 591,338	$ 575,008	$ 591,338
Recorded Investment, 90 Days and Accruing		2,228
Recorded Investment, 90 Days and Accruing	$ 2,575
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Percentage of loans	98.00%
Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	$ 586,644	570,722
Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	4,694	4,286
30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	1,728	1,540
60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	391	518
Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	2,575	2,228
Construction and land development
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	43,514	47,495	43,514
Construction and land development | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	43,514	47,495
Construction and land development | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	9,077	10,471	9,077
Construction and land development | Residential | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	9,077	10,471
Construction and land development | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	34,437	37,024	34,437
Construction and land development | Commercial | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	34,437	37,024
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	305,215	290,590	305,215
Commercial real estate | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	305,215	290,590
Commercial real estate | Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	121,429	122,666	121,429
Commercial real estate | Owner occupied | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	121,429	122,666
Commercial real estate | Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	165,508	154,855	165,508
Commercial real estate | Non-owner occupied | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	165,508	154,855
Commercial real estate | Multifamily
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	18,254	12,743	18,254
Commercial real estate | Multifamily | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	18,254	12,743
Commercial real estate | Farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	24	326	24
Commercial real estate | Farmland | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	24	326
Consumer real estate secured by 1-4 family residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	129,631	128,532	129,631
Consumer real estate secured by 1-4 family residential | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	129,210	128,391
Consumer real estate secured by 1-4 family residential | Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	421	141
Consumer real estate secured by 1-4 family residential | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	421	116
Consumer real estate secured by 1-4 family residential | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans		25
Consumer real estate secured by 1-4 family residential | Home equity lines
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	21,682	21,557	21,682
Consumer real estate secured by 1-4 family residential | Home equity lines | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	21,435	21,449
Consumer real estate secured by 1-4 family residential | Home equity lines | Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	247	108
Consumer real estate secured by 1-4 family residential | Home equity lines | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	247	83
Consumer real estate secured by 1-4 family residential | Home equity lines | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans		25
Consumer real estate secured by 1-4 family residential | First deed of trust
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	95,994	95,638	95,994
Consumer real estate secured by 1-4 family residential | First deed of trust | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	95,820	95,638
Consumer real estate secured by 1-4 family residential | First deed of trust | Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	174
Consumer real estate secured by 1-4 family residential | First deed of trust | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	174
Consumer real estate secured by 1-4 family residential | Second deed of trust
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	11,955	11,337	11,955
Consumer real estate secured by 1-4 family residential | Second deed of trust | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	11,955	11,304
Consumer real estate secured by 1-4 family residential | Second deed of trust | Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans		33
Consumer real estate secured by 1-4 family residential | Second deed of trust | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans		33
Commercial and industrial loans | Commercial and industrial loans (except those secured by real estate)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	92,600	86,203	92,600
Recorded Investment, 90 Days and Accruing	375
Commercial and industrial loans | Commercial and industrial loans (except those secured by real estate) | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	91,457	86,203
Commercial and industrial loans | Commercial and industrial loans (except those secured by real estate) | Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	1,143
Commercial and industrial loans | Commercial and industrial loans (except those secured by real estate) | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	719
Commercial and industrial loans | Commercial and industrial loans (except those secured by real estate) | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	49
Commercial and industrial loans | Commercial and industrial loans (except those secured by real estate) | Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	375
Commercial and industrial loans | Guaranteed student loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	15,782	17,923	15,782
Recorded Investment, 90 Days and Accruing		2,228
Recorded Investment, 90 Days and Accruing	2,200
Commercial and industrial loans | Guaranteed student loans | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	12,652	14,512
Commercial and industrial loans | Guaranteed student loans | Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	3,130	3,411
Commercial and industrial loans | Guaranteed student loans | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	588	690
Commercial and industrial loans | Guaranteed student loans | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	342	493
Commercial and industrial loans | Guaranteed student loans | Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	2,200	2,228
Commercial and industrial loans | Consumer and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	4,596	4,265	$ 4,596
Commercial and industrial loans | Consumer and other | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	$ 4,596	3,531
Commercial and industrial loans | Consumer and other | Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans		734
Commercial and industrial loans | Consumer and other | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans		$ 734