Disclosures about Fair Value of Assets and Liabilities - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($)	Mar. 31, 2025	Sep. 30, 2024
Financial assets:
Held-to-maturity securities	$ 77,760	$ 92,840
Bank owned life insurance	1,852,584	1,829,180
Recurring \| Reported Value Measurement
Financial assets:
Cash and cash equivalents	6,709,669	5,897,596
Interest-bearing time deposits	100,000	100,000
Held-to-maturity securities	77,760	92,840
Loans held for sale	9,483,458	10,538,902
Loans, net	144,465,526	145,078,367
FHLB Stock	1,952,600	1,872,500
Bank owned life insurance	1,852,584	1,829,180
Accrued interest receivable	677,024	685,515
Financial liabilities:
Deposits	138,789,245	134,556,957
FHLB advances		22,000,000
Accrued interest payable	243,488	157,006
Recurring \| Reported Value Measurement \| FHLB advances - short term
Financial liabilities:
FHLB advances	17,000,000
Recurring \| Estimate of Fair Value Measurement
Financial assets:
Cash and cash equivalents	6,709,669	5,897,596
Interest-bearing time deposits	100,000	100,000
Held-to-maturity securities	76,397	91,402
Loans held for sale	10,005,000	10,679,333
Loans, net	132,054,000	135,120,682
FHLB Stock	1,952,600	1,872,500
Bank owned life insurance	1,852,584	1,829,180
Accrued interest receivable	677,024	685,515
Financial liabilities:
Deposits	138,144,985	137,200,002
FHLB advances		21,993,000
Accrued interest payable	243,488	157,006
Recurring \| Estimate of Fair Value Measurement \| FHLB advances - short term
Financial liabilities:
FHLB advances	17,001,000
Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	6,709,669	5,897,596
Interest-bearing time deposits	100,000	100,000
Bank owned life insurance	1,852,584	1,829,180
Accrued interest receivable	677,024	685,515
Financial liabilities:
Deposits	91,905,985	91,699,002
Accrued interest payable	243,488	157,006
Recurring \| Significant Other Observable Inputs (Level 2)
Financial assets:
Held-to-maturity securities	76,397	91,402
FHLB Stock	1,952,600	1,872,500
Financial liabilities:
FHLB advances		21,993,000
Recurring \| Significant Other Observable Inputs (Level 2) \| FHLB advances - short term
Financial liabilities:
FHLB advances	17,001,000
Recurring \| Significant Unobservable Inputs (Level 3)
Financial assets:
Loans held for sale	10,005,000	10,679,333
Loans, net	132,054,000	135,120,682
Financial liabilities:
Deposits	$ 46,239,000	$ 45,501,000

Mercer Bancorp, Inc. 10-Q