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Loans (Tier Risk Rating System) - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2020
Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Repayment period of loan with a reasonable reduction of principal balance	2 years
Good [Member] | Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Leverage position	1.50%
Satisfactory [Member] | Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Leverage position	2.00%

X

- Definition

Leverage ratio.

+ References

No definition available.

+ Details

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Namespace Prefix:	fmao_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Repayment period of loan with reasonable reduction of principal balance.

+ References

No definition available.

+ Details

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Namespace Prefix:	fmao_
Data Type:	xbrli:durationItemType
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_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=fmao_GoodMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=fmao_SatisfactoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: