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Federal Funds Purchased and Securities Sold Under Agreements to Repurchase - Additional Information (Detail) - USD ($)	Sep. 30, 2025	Dec. 31, 2024
Fees And Commissions Income [Abstract]
Federal funds purchased	$ 0	$ 0

X

- Definition

Fees And Commissions Income [Abstract]

+ References

No definition available.

+ Details

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Namespace Prefix:	fmao_
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Period Type:	duration

X

- Definition

Amount of short term borrowing where a bank borrows, at the federal funds rate, from another bank.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13)(1))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

+ Details

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Balance Type:	credit
Period Type:	instant