Debt instrument interest rate reduction percentage.

No definition available.

Name:	fmnb_DebtInstrumentInterestRateReductionPercentage
Namespace Prefix:	fmnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt instrument maturity date length of extension period.

+ References

No definition available.

+ Details

Name:	fmnb_DebtInstrumentMaturityDateLengthOfExtensionPeriod
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

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

- Definition

Provision (reversal) for loan losses expensed.

+ References

No definition available.

+ Details

Name:	fmnb_ProvisionReversalForLoanLossesExpensed
Namespace Prefix:	fmnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
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

Name:	us-gaap_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
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

Name:	us-gaap_FinancingReceivableModificationsRecordedInvestment
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

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
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

Name:	us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown
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

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

- Details

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

- 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
Namespace Prefix:
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
Balance Type:
Period Type:

- Details

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

Troubled Debt Restructurings (Details Textual)	3 Months Ended			6 Months Ended		12 Months Ended
Troubled Debt Restructurings (Details Textual)	Jun. 30, 2017 USD ($) Loans	Jun. 30, 2016 USD ($)	Mar. 31, 2016 Loan Loans	Jun. 30, 2017 USD ($) Loans	Jun. 30, 2016 USD ($) Loan Loans	Dec. 31, 2016 USD ($)
Financing Receivable Modifications [Line Items]
Total troubled debt restructurings	$ 5,800,000			$ 5,800,000		$ 7,000,000
Specific reserves to customers	92,000			92,000		249,000
Commitments to lend additional amounts to borrowers classified as troubled debt restructurings	0			0		0
Provision for loan losses	$ 950,000	$ 990,000		$ 2,000,000	$ 1,770,000	0
Troubled debt restructured loans with subsequent payment defaults \| Loans \| Loans	0			0
Residential Real Estate Loan And Home Equity Line Of Credit
Financing Receivable Modifications [Line Items]
Loans default payment past due period				30 days
Commercial loans
Financing Receivable Modifications [Line Items]
Troubled debt restructured loans with subsequent payment defaults \| Loans \| Loan			2		2
Loans default payment past due period				30 days
Residential real estate loan
Financing Receivable Modifications [Line Items]
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]
Troubled debt restructured loans with subsequent payment defaults \| Loans \| Loans			1		1
Troubled debt restructurings
Financing Receivable Modifications [Line Items]
Specific reserves to customers	$ 92,000			$ 92,000		$ 101,000
Allowance adjustment charge offs	17,000	316,000		30,000	$ 327,000
Provision for loan losses	$ 17,000	$ 0		$ 30,000	$ 11,000
Contractual Interest Rate Reduction
Financing Receivable Modifications [Line Items]
Reduction of the notes stated interest rate		1.24%			1.24%
Contractual Interest Rate Reduction \| Minimum
Financing Receivable Modifications [Line Items]
Reduction of the notes stated interest rate	0.49%			0.49%
Contractual Interest Rate Reduction \| Maximum
Financing Receivable Modifications [Line Items]
Reduction of the notes stated interest rate	1.89%			1.89%
Extended Maturity
Financing Receivable Modifications [Line Items]
Maturity period loans					120 months
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