Loans and allowance for loan losses - Risk category of loans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	$ 25,465
2022	91,092
2021	97,295
2020	91,832
2019	47,596
Prior	169,121
Revolving	68,937
Total loans	591,338	$ 575,008	$ 591,338
Current-period gross write-offs
2023	6
Construction and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	43,514	47,495	43,514
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	305,215	290,590	305,215
Consumer real estate secured by 1-4 family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	129,631	128,532	129,631
Residential | Construction and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	876
2022	5,656
2021	2,206
2020	339
Total loans	9,077	10,471	9,077
Residential | Construction and land development | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	876
2022	5,656
2021	2,206
2020	339
Total loans			9,077
Commercial | Construction and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	692
2022	6,649
2021	14,193
2020	10,331
2019	224
Prior	1,081
Revolving	1,267
Total loans	34,437	37,024	34,437
Commercial | Construction and land development | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	692
2022	6,649
2021	14,193
2020	10,331
2019	224
Prior	1,081
Revolving	1,267
Total loans			34,437
Owner occupied | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	666
2022	12,518
2021	21,838
2020	19,220
2019	9,737
Prior	56,876
Revolving	574
Total loans	121,429	122,666	121,429
Owner occupied | Commercial real estate | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	666
2022	12,518
2021	21,638
2020	19,149
2019	9,276
Prior	52,199
Revolving	574
Total loans			116,020
Owner occupied | Commercial real estate | Special Mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	200
2020	71
2019	461
Prior	4,677
Total loans			5,409
Non-owner occupied | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	6,692
2022	9,425
2021	25,419
2020	30,403
2019	23,337
Prior	60,376
Revolving	9,856
Total loans	165,508	154,855	165,508
Non-owner occupied | Commercial real estate | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	6,692
2022	9,425
2021	25,419
2020	28,243
2019	23,337
Prior	57,626
Revolving	9,856
Total loans			160,598
Non-owner occupied | Commercial real estate | Special Mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	2,160
Prior	2,750
Total loans			4,910
Multifamily | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	5,250
2022	1,300
2020	2,434
2019	542
Prior	6,857
Revolving	1,871
Total loans	18,254	12,743	18,254
Multifamily | Commercial real estate | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	5,250
2022	1,300
2020	2,434
2019	542
Prior	6,857
Revolving	1,871
Total loans			18,254
Farmland | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Prior	24
Total loans	24	326	24
Farmland | Commercial real estate | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Prior	24
Total loans			24
Home equity lines | Consumer real estate secured by 1-4 family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	445
Revolving	21,237
Total loans	21,682	21,557	21,682
Home equity lines | Consumer real estate secured by 1-4 family residential | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	445
Revolving	21,162
Total loans			21,607
Home equity lines | Consumer real estate secured by 1-4 family residential | Special Mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revolving	75
Total loans			75
First deed of trust | Consumer real estate secured by 1-4 family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	4,330
2022	32,416
2021	14,608
2020	14,152
2019	8,006
Prior	20,358
Revolving	2,124
Total loans	95,994	95,638	95,994
First deed of trust | Consumer real estate secured by 1-4 family residential | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	4,330
2022	32,416
2021	14,608
2020	14,152
2019	8,006
Prior	19,985
Revolving	2,124
Total loans			95,621
First deed of trust | Consumer real estate secured by 1-4 family residential | Special Mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Prior	214
Total loans			214
First deed of trust | Consumer real estate secured by 1-4 family residential | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Prior	159
Total loans			159
Second deed of trust | Consumer real estate secured by 1-4 family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	681
2022	4,485
2021	3,088
2020	1,009
2019	388
Prior	1,782
Revolving	522
Total loans	11,955	11,337	11,955
Second deed of trust | Consumer real estate secured by 1-4 family residential | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	593
2022	4,485
2021	3,088
2020	1,009
2019	388
Prior	1,572
Revolving	522
Total loans			11,657
Second deed of trust | Consumer real estate secured by 1-4 family residential | Special Mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	88
Prior	110
Total loans			198
Second deed of trust | Consumer real estate secured by 1-4 family residential | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Prior	100
Total loans			100
Commercial and industrial loans (except those secured by real estate) | Commercial and industrial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	6,036
2022	18,219
2021	15,095
2020	13,837
2019	5,324
Prior	5,963
Revolving	28,126
Total loans	92,600	86,203	92,600
Commercial and industrial loans (except those secured by real estate) | Commercial and industrial loans | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	6,036
2022	18,185
2021	15,095
2020	13,837
2019	5,313
Prior	5,903
Revolving	27,757
Total loans			92,126
Commercial and industrial loans (except those secured by real estate) | Commercial and industrial loans | Special Mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	34
Prior	49
Revolving	369
Total loans			452
Commercial and industrial loans (except those secured by real estate) | Commercial and industrial loans | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
2019	11
Prior	11
Total loans			22
Guaranteed student loans | Commercial and industrial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Prior	15,782
Total loans	15,782	17,923	15,782
Current-period gross write-offs
2023	6
Guaranteed student loans | Commercial and industrial loans | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Prior	15,782
Total loans			15,782
Consumer and other | Commercial and industrial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	242
2022	424
2021	403
2020	107
2019	38
Prior	22
Revolving	3,360
Total loans	4,596	$ 4,265	4,596
Consumer and other | Commercial and industrial loans | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
2023	242
2022	424
2021	403
2020	107
2019	38
Prior	22
Revolving	$ 3,360
Total loans			$ 4,596