Debt Securities - Continuous Unrealized Loss Positions (Details) - USD ($)	Dec. 31, 2025	Sep. 30, 2025
Available for sale, Fair Value:
Less than 12 Months, Fair Value	$ 617,978	$ 1,975,598
12 Months or More, Fair Value	5,827,895	6,343,981
Total, Fair Value	6,445,873	8,319,579
Available for sale, Unrealized Losses:
Less than 12 Months, Unrealized Losses	(3,565)	(83,628)
12 Months or More, Unrealized Losses	(449,565)	(453,877)
Total, Unrealized Losses	(453,130)	(537,505)
Held to maturity, Unrealized Losses:
Less than 12 Months, Fair Value	617,978	1,975,598
12 Months or More, Fair Value	5,890,821	6,411,161
Total, Fair Value	6,508,799	8,386,759
Less than 12 Months, Unrealized Losses	(3,565)	(454,420)
12 Months or More, Unrealized Losses	(450,074)	(83,628)
Total, Unrealized Losses	(453,639)	(538,048)
U.S. Government agencies
Available for sale, Fair Value:
Less than 12 Months, Fair Value	617,978
12 Months or More, Fair Value		1,994,040
Total, Fair Value	617,978	1,994,040
Available for sale, Unrealized Losses:
Less than 12 Months, Unrealized Losses	(3,565)
12 Months or More, Unrealized Losses		(5,960)
Total, Unrealized Losses	(3,565)	(5,960)
Mortgage-backed Government Sponsored Enterprises (GSEs)
Available for sale, Fair Value:
Less than 12 Months, Fair Value		560,974
12 Months or More, Fair Value	2,811,650	2,418,879
Total, Fair Value	2,811,650	2,979,853
Available for sale, Unrealized Losses:
Less than 12 Months, Unrealized Losses		(15,975)
12 Months or More, Unrealized Losses	(163,788)	(172,462)
Total, Unrealized Losses	(163,788)	(188,437)
Held to maturity, Fair Value:
12 Months or More, Fair Value	62,926	67,180
Total, Fair Value	62,926	67,180
Held to maturity, Unrealized Losses:
12 Months or More, Unrealized Losses	(509)	(543)
Total, Unrealized Losses	(509)	(543)
State and political subdivisions
Available for sale, Fair Value:
Less than 12 Months, Fair Value		1,414,624
12 Months or More, Fair Value	3,016,245	1,931,062
Total, Fair Value	3,016,245	3,345,686
Available for sale, Unrealized Losses:
Less than 12 Months, Unrealized Losses		(67,653)
12 Months or More, Unrealized Losses	(285,777)	(275,455)
Total, Unrealized Losses	$ (285,777)	$ (343,108)

Mercer Bancorp, Inc. 10-Q