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Business Combination and Asset Purchase - Summary of Carrying Amount of Loans (Parenthetical) (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Jan. 01, 2019
Business Combinations [Abstract]
Fair value adjustment	$ 0	$ 62	$ 2,118

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- Definition

Certain loans acquired in transfer accounted for as debt securities fair value adjustment.

+ References

No definition available.

+ Details

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Namespace Prefix:	fmao_
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Balance Type:	debit
Period Type:	instant

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- References

No definition available.

+ Details

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