Deposits - Time Deposits (Detail) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Deposits [Abstract]
Time deposits under $250,000	$ 505,949	$ 523,895
Time deposits of $250,000 or more	141,632	139,122
Total	$ 647,581	$ 663,017

Amount of time deposit liabilities, including certificates of deposit, in denominations of less than $250,000.

No definition available.

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Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
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Period Type:	duration

- Definition

Amount of time deposit liabilities, including certificates of deposit.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-SubTopic 405
-Topic 942
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478353/942-405-50-1

Reference 2: 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(12))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

+ Details

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- Definition

Amount of time deposit liabilities, including certificates of deposit, in denominations that meet or exceed the Federal Deposit Insurance Corporation (FDIC) insurance limit.

+ References

+ Details

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Namespace Prefix:	us-gaap_
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