Fair Value (Details Textual) (USD $)	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Impaired loans carried at fair value	$ 451,000		$ 988,000
Impaired loans collateral dependent loans valuation allowance	42,000		117,000
Additional provision for loan losses	215,000fmnb_AdditionalProvisionForLoanLoss	0fmnb_AdditionalProvisionForLoanLoss
Troubled debt restructurings not carried at fair value	$ 4,100,000		$ 4,200,000
Maximum maturity period of short term borrowings	90 days

Additional provision for loan loss.

No definition available.

Name:	fmnb_AdditionalProvisionForLoanLoss
Namespace Prefix:	fmnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Impaired financing receivable collateral dependent loans unpaid principal balance.

+ References

No definition available.

+ Details

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

- Definition

Impaired financing receivable collateral dependent loans valuation allowance.

+ References

No definition available.

+ Details

Name:	fmnb_ImpairedFinancingReceivableCollateralDependentLoansValuationAllowance
Namespace Prefix:	fmnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FairValueDisclosuresAbstract
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=25495931&loc=d3e18845-111554

+ Details

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