Principles of presentation - Impact of ASC 326 (Details) - USD ($)	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
Allowance Ending Balance	$ 3,574,000	$ 3,423,000		$ 3,270,000	$ 3,370,000
Total Allowance for credit losses	3,574,000	3,423,000		3,270,000	3,370,000
Allowance for credit losses on unfunded credit exposure	320,000			254,000
Total Allowance for credit losses	3,890,000			3,530,000
Retained earnings	13,278,000	11,775,000
Construction and land development
Allowance Ending Balance	259,000	314,000	$ 259,000	315,000	271,000
Total Allowance for credit losses	259,000	314,000	259,000	315,000	271,000
Construction and land development | Residential
Allowance Ending Balance	57,000	86,000		51,000	79,000
Total Allowance for credit losses	57,000	86,000		51,000	79,000
Construction and land development | Commercial
Allowance Ending Balance	202,000	228,000		264,000	192,000
Total Allowance for credit losses	202,000	228,000		264,000	192,000
Commercial real estate
Allowance Ending Balance	2,119,000	1,923,000	2,119,000	1,891,000	2,189,000
Total Allowance for credit losses	2,119,000	1,923,000	2,119,000	1,891,000	2,189,000
Commercial real estate | Owner occupied
Allowance Ending Balance	437,000	409,000		391,000	867,000
Total Allowance for credit losses	437,000	409,000		391,000	867,000
Commercial real estate | Non-owner occupied
Allowance Ending Balance	1,596,000	1,467,000		1,460,000	1,289,000
Total Allowance for credit losses	1,596,000	1,467,000		1,460,000	1,289,000
Commercial real estate | Multifamily
Allowance Ending Balance	86,000	44,000		40,000	33,000
Total Allowance for credit losses	86,000	44,000		40,000	33,000
Commercial real estate | Farmland
Allowance Ending Balance		3,000
Total Allowance for credit losses		3,000
Consumer real estate secured by 1-4 family residential
Allowance Ending Balance	428,000	432,000	428,000	322,000	185,000
Total Allowance for credit losses	428,000	432,000	$ 428,000	322,000	185,000
Consumer real estate secured by 1-4 family residential | Home equity lines
Allowance Ending Balance	32,000	40,000		33,000	11,000
Total Allowance for credit losses	32,000	40,000		33,000	11,000
Consumer real estate secured by 1-4 family residential | First deed of trust
Allowance Ending Balance	298,000	293,000		214,000	131,000
Total Allowance for credit losses	298,000	293,000		214,000	131,000
Consumer real estate secured by 1-4 family residential | Second deed of trust
Allowance Ending Balance	98,000	99,000		75,000	43,000
Total Allowance for credit losses	98,000	99,000		75,000	43,000
Commercial and industrial loans
Allowance Ending Balance				3,272,000	3,370,000
Total Allowance for credit losses				3,272,000	3,370,000
Commercial and industrial loans | Commercial and industrial loans (except those secured by real estate)
Allowance Ending Balance	666,000	640,000		549,000	576,000
Total Allowance for credit losses	666,000	640,000		549,000	576,000
Commercial and industrial loans | Guaranteed student loans
Allowance Ending Balance	55,000	57,000		112,000	52,000
Total Allowance for credit losses	55,000	57,000		112,000	52,000
Commercial and industrial loans | Consumer and other
Allowance Ending Balance	36,000	36,000		34,000	37,000
Total Allowance for credit losses	36,000	36,000		34,000	37,000
Commercial and industrial loans | Unallocated
Allowance Ending Balance	11,000	21,000		49,000	60,000
Total Allowance for credit losses	$ 11,000	$ 21,000		$ 49,000	$ 60,000