Commercial loan and commercial real estate risk maximum.

No definition available.

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Namespace Prefix:	fmnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Debt instrument interest rate reduction percentage.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt instrument maturity date length of extension period.

+ References

No definition available.

+ Details

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Namespace Prefix:	fmnb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Financing receivable period of modified contractually past due loans to be considered as payment default.

+ References

No definition available.

+ Details

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Namespace Prefix:	fmnb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of allowance for credit losses related to receivables which had been measured under a general allowance for credit losses methodology that are now identified as troubled debt restructurings.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18845-111554

+ Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953682-111524

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Reflects the amount of the write-down related to a troubled debt restructuring.

+ References

+ Details

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

- Definition

Reflects the unfunded portion of contractual agreements to lend funds to borrowers for loans whose terms have been modified by troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18794-111554

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageMember
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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=fmnb_ResidentialRealEstateHomeEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=fmnb_TroubledDebtRestructuringsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ContractualInterestRateReductionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
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- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
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Loans (Details Textual)	12 Months Ended
Loans (Details Textual)	Dec. 31, 2017 USD ($) Loans	Dec. 31, 2016 USD ($) Loans	Dec. 31, 2015 USD ($) Loans
Financing Receivable Modifications [Line Items]
Total troubled debt restructurings	$ 5,000,000	$ 7,000,000
Specific reserves to customers	162,000	249,000
Commitments to lend additional amounts to borrowers classified as troubled debt restructurings	0	0
Allowance adjustment charge offs	75,000	344,000	$ 129,000
Increase in the allowance for loan losses as a result of the allowance	$ 75,000	43,000	101,000
Troubled debt restructured loans with subsequent payment defaults \| Loans \| Loans	0
Loans default payment past due period	30 days
Maximum commercial loan and commercial real estate relationships	$ 750,000
Commercial real estate loan
Financing Receivable Modifications [Line Items]
Total troubled debt restructurings		$ 1,200,000	$ 40,000
Troubled debt restructured loans with subsequent payment defaults \| Loans \| Loans		2	1
Residential real estate loan
Financing Receivable Modifications [Line Items]
Total troubled debt restructurings		$ 1,000	$ 1,000
Troubled debt restructured loans with subsequent payment defaults \| Loans \| Loans		1	1
Residential real estate, Home equity lines of credit
Financing Receivable Modifications [Line Items]
Total troubled debt restructurings		$ 10,000	$ 11,000
Troubled debt restructured loans with subsequent payment defaults \| Loans \| Loans		1	1
Troubled debt restructurings
Financing Receivable Modifications [Line Items]
Specific reserves to customers	$ 68,000	$ 101,000
Contractual Interest Rate Reduction \| Minimum
Financing Receivable Modifications [Line Items]
Reduction of the notes stated interest rate	0.49%
Contractual Interest Rate Reduction \| Maximum
Financing Receivable Modifications [Line Items]
Reduction of the notes stated interest rate	11.51%
Extended Maturity \| Minimum
Financing Receivable Modifications [Line Items]
Maturity period loans	6 months
Extended Maturity \| Maximum
Financing Receivable Modifications [Line Items]
Maturity period loans	132 months