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.

No definition available.

+ Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fmao_AgricultureRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fmao_AgriculturalLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans - Impaired Loans Classified as Troubled Debt Restructured (Detail) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($) Contract	Jun. 30, 2019 USD ($) Contract	Jun. 30, 2020 USD ($) Contract Loan	Jun. 30, 2019 USD ($) Contract	Dec. 31, 2019 Loan
Financing Receivable Modifications [Line Items]
Number of Contracts Modified in the Last three Months \| Loan			5		4
Commercial Real Estate [Member]
Financing Receivable Modifications [Line Items]
Number of Contracts Modified in the Last three Months \| Contract			2
Pre-Modification Outstanding Recorded Investment			$ 981
Post-Modification Outstanding Recorded Investment			$ 981
Ag Real Estate [Member]
Financing Receivable Modifications [Line Items]
Number of Contracts Modified in the Last three Months \| Contract	2		2
Pre-Modification Outstanding Recorded Investment	$ 5,380		$ 5,380
Post-Modification Outstanding Recorded Investment	$ 5,380		$ 5,380
Commercial and Industrial [Member]
Financing Receivable Modifications [Line Items]
Number of Contracts Modified in the Last three Months \| Contract		4		4
Pre-Modification Outstanding Recorded Investment		$ 812		$ 812
Post-Modification Outstanding Recorded Investment		$ 812		$ 812
Agricultural [Member]
Financing Receivable Modifications [Line Items]
Number of Contracts Modified in the Last three Months \| Contract	1		1
Pre-Modification Outstanding Recorded Investment	$ 164		$ 164
Post-Modification Outstanding Recorded Investment	$ 164		$ 164