Debt instrument interest rate reduction percentage.

No definition available.

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- Definition

Debt instrument maturity date length of extension period.

+ References

No definition available.

+ Details

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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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Data Type:	xbrli:durationItemType
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

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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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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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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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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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

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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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Troubled Debt Restructurings (Details Textual)	3 Months Ended		9 Months Ended		12 Months Ended
Troubled Debt Restructurings (Details Textual)	Sep. 30, 2017 USD ($) Loans	Sep. 30, 2016 USD ($) Loan Loans	Sep. 30, 2017 USD ($) Loans	Sep. 30, 2016 USD ($) Loan Loans	Dec. 31, 2016 USD ($)
Financing Receivable Modifications [Line Items]
Total troubled debt restructurings	$ 4,900,000		$ 4,900,000		$ 7,000,000
Specific reserves to customers	77,000		77,000		249,000
Commitments to lend additional amounts to borrowers classified as troubled debt restructurings	0		0		0
Provision for loan losses	$ 950,000	$ 1,110,000	$ 2,950,000	$ 2,880,000	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	$ 77,000		$ 77,000		$ 101,000
Allowance adjustment charge offs	30,000	$ 25,000	60,000	$ 353,000
Provision for loan losses	$ 30,000	$ 25,000	$ 60,000	$ 36,000
Contractual Interest Rate Reduction
Financing Receivable Modifications [Line Items]
Reduction of the notes stated interest rate		1.20%		1.20%
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