Capital required for capital adequacy to risk weighted assets including capital conservation buffer.

No definition available.

Name:	fmao_CapitalRequiredForCapitalAdequacyToRiskWeightedAssetsIncludingCapitalConservationBuffer
Namespace Prefix:	fmao_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Risk weighted assets capital conservation buffer.

+ References

No definition available.

+ Details

Name:	fmao_RiskWeightedAssetsCapitalConservationBuffer
Namespace Prefix:	fmao_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of minimum total risk-based capital to risk-weighted assets required for capital adequacy as defined by regulatory framework.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477270/942-505-50-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477270/942-505-50-1B

+ Details

Name:	us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- 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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477270/942-505-50-1B

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477270/942-505-50-1

+ Details

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

- Definition

Ratio of Tier 1 capital to average assets as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_StatementClassOfStockAxis=fmao_CommonEquityTierOneCapitalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=fmao_TierOneCapitalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Minimum Regulatory Capital Requirements - Additional Information (Detail)	Dec. 31, 2024	Dec. 31, 2023	Jan. 01, 2019
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Minimum ratio of capital to risk-weighted assets	0.08	0.08	0.080
Minimum ratio of capital to risk-weighted assets including capital conservation buffer			10.50%
Minimum leverage ratio calculated as ratio of Tier 1 capital to average quarterly assets	0.0812	0.0786	0.040
Common Equity Tier 1 Capital [Member]
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Minimum ratio of capital to risk-weighted assets			0.045
Capital conservation buffer			2.50%
Minimum ratio of capital to risk-weighted assets including capital conservation buffer			7.00%
Tier 1 Capital [Member]
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Minimum ratio of capital to risk-weighted assets			0.060
Minimum ratio of capital to risk-weighted assets including capital conservation buffer			8.50%