Credit Quality Indicators (Details Textual) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015
Credit Quality Indicators [Abstract]
Maximum commercial loan and commercial real estate relationships	$ 750

Commercial loan and commercial real estate risk maximum.

No definition available.

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