Financing receivable modification specific reserve to customer.

No definition available.

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

- Definition

Number of loans charged off.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Provision (reversal) for loan losses expensed.

+ References

No definition available.

+ Details

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

- Definition

Troubled debt restructuring commitment to lend additional amount to borrow.

+ References

No definition available.

+ Details

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

- Definition

Troubled debt restructurings, modification of loan terms included reduction of stated interest rate, remaining life.

+ References

No definition available.

+ Details

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

- Definition

Troubled debt restructurings payment default.

+ References

No definition available.

+ Details

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

- Definition

Amount of increase (decrease) of allowance reserve for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(d))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph d

-Article 9

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

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

- 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=35738203&loc=SL6953682-111524

+ Details

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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

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

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

- Definition

Amount of expense related to loss from loan transactions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 11

-Article 9

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.11)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

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

- Definition

Threshold period past due for financing receivable to write off as uncollectible, 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_ThresholdPeriodPastDueForWriteoffOfFinancingReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

For a debtor, the aggregate gain (loss) recognized on the restructuring of payables, after the impact of income tax expense or benefit, arising from the difference between the book value of the debt before the restructuring and the fair value of the payments on the debt after restructuring is complete.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 60

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6402221&loc=d3e15743-112638

+ Details

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

- Details

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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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Troubled Debt Restructurings (Details Textual) (USD $)	3 Months Ended		12 Months Ended
Troubled Debt Restructurings (Details Textual) (USD $)	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Financing Receivable Modifications [Line Items]
Total troubled debt restructurings	$ 8,200,000		$ 8,100,000
Specific reserves to customers	247,000		242,000
Commitments to lend additional amounts to borrowers classified as troubled debt restructurings	11,000		25,000
Allowance adjustment charge offs	35,000	0
Increase in the allowance for loan losses as a result of the allowance	33,000	26,000
Additional provision impact to allowances for loan losses	450,000	330,000
Residential Real Estate
Financing Receivable Modifications [Line Items]
Loans modified as trouble debt restructuring loan	1
Payment default loan period	12 months
Additional provision impact to allowances for loan losses	23,000
Loans default payment past due period	30 days
Additional provision impact to allowances for loan losses	10,000	(228,000)
Consumer, Indirect
Financing Receivable Modifications [Line Items]
Loans modified as trouble debt restructuring loan		4
Payment default loan period		12 months
Loans default payment past due period	30 days
Additional provision impact to allowances for loan losses	$ 1,000
Loan charged off		1
Minimum
Financing Receivable Modifications [Line Items]
Reduction of the notes stated interest rate	0.87%
Maturity period loans	9 months
Maximum
Financing Receivable Modifications [Line Items]
Reduction of the notes stated interest rate	2.75%
Maturity period loans	120 months