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Business Combination and Asset Purchase - Summary of Consideration Paid and Amounts of Assets Acquired and Liabilities Assumed Recognized (Parenthetical) (Detail)	Nov. 16, 2020 shares
Adams County Financial Resources [Member]
Business Acquisition [Line Items]
Common stock issued in exchange of each outstanding share	40,049

X

- Definition

Number of shares of equity interests issued or issuable to acquire entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)(4)
-URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479

+ Details

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Balance Type:	na
Period Type:	duration

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=fmao_AdamsCountyFinancialResourcesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: