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Restrictions of Dividends & Inter-company Borrowings - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2022 USD ($)
Class of Stock [Line Items]
Dividends may be paid without prior regulatory approval	$ 42.9
Perpetual Federal Savings Bank [Member]
Class of Stock [Line Items]
Dividend approved by federal reserve for the acquisition	$ 40.0

X

- Definition

Dividends may be paid without prior regulatory approval.

+ References

No definition available.

+ Details

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

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

X

- Definition

Amount of dividends that can be declared or paid with approval of regulatory agency.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: