Loans - Risk Category of Loans at Amortized Cost, by Portfolio Segment Based on Year of Origination (Detail) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Financing Receivable, Credit Quality Indicator [Line Items]
Grand Total	$ 2,580,192	$ 2,560,795
Consumer Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	15,212	40,400
2024	38,410	64,555
2023	59,974	80,069
2022	78,318	91,137
2021	89,619	75,461
Prior	177,404	107,166
Term Total	458,937	458,788
Revolving Loans Amortized Cost Basis	64,447	61,352
Revolving Loans Converted to Term	320	252
Grand Total	523,704	520,392
Gross charge-offs - Prior		13
Gross charge-offs - Term Total		13
Gross charge-offs - Grand Total		13
Consumer Real Estate [Member] \| Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	15,212	40,257
2024	38,267	64,316
2023	59,741	79,503
2022	77,759	89,800
2021	88,204	74,996
Prior	175,384	106,007
Term Total	454,567	454,879
Revolving Loans Amortized Cost Basis	64,194	61,097
Revolving Loans Converted to Term	305	237
Grand Total	519,066	516,213
Consumer Real Estate [Member] \| Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023		37
2022	36	551
2021	135
Prior	117	119
Term Total	288	707
Revolving Loans Amortized Cost Basis	19	19
Grand Total	307	726
Consumer Real Estate [Member] \| Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		143
2024	143	239
2023	233	529
2022	523	786
2021	1,280	465
Prior	1,903	1,040
Term Total	4,082	3,202
Revolving Loans Amortized Cost Basis	234	236
Revolving Loans Converted to Term	15	15
Grand Total	4,331	3,453
Agricultural Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	6,366	29,513
2024	28,100	28,088
2023	26,912	34,840
2022	34,032	22,983
2021	22,633	23,639
Prior	97,525	76,982
Term Total	215,568	216,045
Revolving Loans Amortized Cost Basis	88	92
Grand Total	215,656	216,137
Agricultural Real Estate [Member] \| Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	6,366	23,817
2024	22,431	28,088
2023	26,912	34,469
2022	33,661	22,983
2021	22,633	23,639
Prior	97,422	76,964
Term Total	209,425	209,960
Revolving Loans Amortized Cost Basis	88	92
Grand Total	209,513	210,052
Agricultural Real Estate [Member] \| Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Prior	12	13
Term Total	12	13
Grand Total	12	13
Agricultural Real Estate [Member] \| Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		5,696
2024	5,669
2023		371
2022	371
Prior	91	5
Term Total	6,131	6,072
Grand Total	6,131	6,072
Agricultural [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	9,944	14,915
2024	12,389	10,534
2023	9,441	14,381
2022	12,229	5,624
2021	4,991	3,233
Prior	6,327	3,911
Term Total	55,321	52,598
Revolving Loans Amortized Cost Basis	98,600	99,783
Grand Total	153,921	152,381
Agricultural [Member] \| Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	9,944	14,915
2024	12,389	10,500
2023	9,423	14,381
2022	11,856	5,616
2021	4,985	3,204
Prior	6,327	3,911
Term Total	54,924	52,527
Revolving Loans Amortized Cost Basis	94,013	98,283
Grand Total	148,937	150,810
Agricultural [Member] \| Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		13
2022	373	8
2021	6
Term Total	379	21
Revolving Loans Amortized Cost Basis	3,116	30
Grand Total	3,495	51
Agricultural [Member] \| Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		21
2023	18
2021		29
Term Total	18	50
Revolving Loans Amortized Cost Basis	1,471	1,470
Grand Total	1,489	1,520
Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	54,325	128,608
2024	145,473	234,491
2023	238,525	425,696
2022	417,882	218,309
2021	215,108	111,787
Prior	251,949	189,570
Term Total	1,323,262	1,308,461
Grand Total	1,323,262	1,308,461
Gross charge-offs - Prior		15
Gross charge-offs - Term Total		15
Gross charge-offs - Grand Total		15
Commercial Real Estate [Member] \| Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	52,969	128,608
2024	145,421	200,192
2023	204,446	413,106
2022	404,021	218,309
2021	214,795	110,435
Prior	249,291	188,239
Term Total	1,270,943	1,258,889
Grand Total	1,270,943	1,258,889
Commercial Real Estate [Member] \| Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	1,356
2023		12,590
2022	12,514
2021	313	1,352
Prior	1,944	753
Term Total	16,127	14,695
Grand Total	16,127	14,695
Commercial Real Estate [Member] \| Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	52	34,299
2023	34,079
2022	1,347
Prior	714	578
Term Total	36,192	34,877
Grand Total	36,192	34,877
Commercial & Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	9,415	31,972
2024	28,325	55,066
2023	47,272	39,694
2022	35,844	15,235
2021	14,005	13,506
Prior	11,339	1,738
Term Total	146,200	157,211
Revolving Loans Amortized Cost Basis	131,665	117,386
Revolving Loans Converted to Term	78	222
Grand Total	277,943	274,819
Gross charge-offs - fiscal year 2022	25
Gross charge-offs - fiscal year 2021		101
Gross charge-offs - Term Total	25	101
Gross charge-offs - Revolving Loans Converted to Term		5
Gross charge-offs - Grand Total	25	106
Commercial & Industrial [Member] \| Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	9,415	31,933
2024	28,287	54,581
2023	46,818	39,665
2022	35,844	15,047
2021	13,786	13,480
Prior	10,925	1,294
Term Total	145,075	156,000
Revolving Loans Amortized Cost Basis	127,302	113,446
Revolving Loans Converted to Term	78	222
Grand Total	272,455	269,668
Commercial & Industrial [Member] \| Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		137
2023	127
2022		188
2021	182	26
Prior	386	416
Term Total	695	767
Revolving Loans Amortized Cost Basis	480	459
Grand Total	1,175	1,226
Commercial & Industrial [Member] \| Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		39
2024	38	348
2023	327	29
2021	37
Prior	28	28
Term Total	430	444
Revolving Loans Amortized Cost Basis	3,883	3,481
Grand Total	4,313	3,925
Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Grand Total	60,721	63,627
Consumer [Member] \| Payment Performance [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	5,760	13,477
2024	11,826	13,521
2023	12,038	27,304
2022	23,741	5,952
2021	4,935	2,314
Prior	2,187	582
Term Total	60,487	63,150
Revolving Loans Amortized Cost Basis	234	477
Grand Total	60,721	63,627
Gross charge-offs - fiscal year 2025	40	201
Gross charge-offs - fiscal year 2024	43	69
Gross charge-offs - fiscal year 2023	19	62
Gross charge-offs - fiscal year 2022	162	14
Gross charge-offs - fiscal year 2021	46
Gross charge-offs - Term Total	310	346
Gross charge-offs - Grand Total	310	346
Consumer [Member] \| Performing [Member] \| Payment Performance [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	5,760	13,437
2024	11,803	13,521
2023	12,036	27,264
2022	23,711	5,917
2021	4,923	2,310
Prior	2,184	582
Term Total	60,417	63,031
Revolving Loans Amortized Cost Basis	234	477
Grand Total	60,651	63,508
Consumer [Member] \| Nonperforming [Member] \| Payment Performance [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		40
2024	23
2023	2	40
2022	30	35
2021	12	4
Prior	3
Term Total	70	119
Grand Total	70	119
Other Loan [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	600
2022		15,829
2021	15,666	5,068
Prior	8,719	4,081
Term Total	24,985	24,978
Grand Total	24,985	24,978
Other Loan [Member] \| Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	600
2022		15,829
2021	15,666	5,068
Prior	8,719	4,081
Term Total	24,985	24,978
Grand Total	$ 24,985	$ 24,978

Financing receivable allowance for credi tlosses writeoffs in current fiscal year.

No definition available.

Name:	fmao_FinancingReceivableAllowanceForCrediTlossesWriteoffsInCurrentFiscalYear
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing receivable allowance for credit losses writeoffs fiscal year before current fiscal year.

+ References

No definition available.

+ Details

Name:	fmao_FinancingReceivableAllowanceForCreditLossesWriteoffsFiscalYearBeforeCurrentFiscalYear
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing Receivable Allowance For Credit Losses Writeoffs Five Years Before Latest Fiscal Year.

+ References

No definition available.

+ Details

Name:	fmao_FinancingReceivableAllowanceForCreditLossesWriteoffsFiveYearsBeforeLatestFiscalYear
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing receivable allowance for credit losses writeoffs four or more years before latest fiscal year.

+ References

No definition available.

+ Details

Name:	fmao_FinancingReceivableAllowanceForCreditLossesWriteoffsFourOrMoreYearsBeforeLatestFiscalYear
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing receivable allowance for credit losses writeoffs grand net.

+ References

No definition available.

+ Details

Name:	fmao_FinancingReceivableAllowanceForCreditLossesWriteoffsGrandNet
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing receivable allowance for credit losses writeoffs revolving loans converted to term.

+ References

No definition available.

+ Details

Name:	fmao_FinancingReceivableAllowanceForCreditLossesWriteoffsRevolvingLoansConvertedToTerm
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing receivable allowance for credit losses writeoffs term total.

+ References

No definition available.

+ Details

Name:	fmao_FinancingReceivableAllowanceForCreditLossesWriteoffsTermTotal
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing receivable allowance for credit losses writeoffs three years before latest fiscal year.

+ References

No definition available.

+ Details

Name:	fmao_FinancingReceivableAllowanceForCreditLossesWriteoffsThreeYearsBeforeLatestFiscalYear
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing receivable allowance for credit losses writeoffs two years before latest fiscal year.

+ References

No definition available.

+ Details

Name:	fmao_FinancingReceivableAllowanceForCreditLossesWriteoffsTwoYearsBeforeLatestFiscalYear
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing receivable term loan.

+ References

No definition available.

+ Details

Name:	fmao_FinancingReceivableTermLoan
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing receivables net of deferred income.

+ References

No definition available.

+ Details

Name:	fmao_FinancingReceivablesNetOfDeferredIncome
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of financing receivable originated more than five years prior to current fiscal year. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479294/326-20-55-79

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-6

+ Details

Name:	us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of financing receivable originated four years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

Name:	us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of financing receivable originated in current fiscal year. Excludes net investment in lease.

+ References

+ Details

Name:	us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of financing receivable originated in fiscal year prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

Name:	us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of financing receivable originated three years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

Name:	us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of financing receivable originated two years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

Name:	us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 29
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-29

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-5

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479294/326-20-55-79

+ Details

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

- Definition

Amortized cost of financing receivable that can be withdrawn, repaid, and redrawn.

+ References

+ Details

Name:	us-gaap_FinancingReceivableRevolving
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of revolving financing receivable converted to term loan.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479294/326-20-55-79

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-6A

+ Details

Name:	us-gaap_FinancingReceivableRevolvingConvertedToTermLoan
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fmao_AgriculturalRealEstateMember
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:

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmao_PaymentPerformanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentPerformanceStatusAxis=us-gaap_PerformingFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentPerformanceStatusAxis=us-gaap_NonperformingFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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