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Interest Bearing Deposits (Details Textual) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Interest Bearing Deposits (Textual) [Abstract]
Minimum time deposits amount	$ 250,000
Time deposits of $100 thousand or more	$ 79,800,000	$ 51,900,000

X

- Definition

Interest Bearing Deposits (Textual) [Abstract]

+ References

No definition available.

+ Details

Name:	fmnb_InterestBearingDepositsTextualAbstract
Namespace Prefix:	fmnb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Minimum time deposits.

+ References

No definition available.

+ Details

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

X

- Definition

Amount of time deposit liabilities, including certificates of deposit, in denominations of $100,000 or more.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher SEC
-Name Industry Guide
-Number 3
-Section V
-Paragraph D-E

+ Details

Name:	us-gaap_TimeDeposits100000OrMore
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant