Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Oct. 31, 2019
Cover [Abstract]
Entity Registrant Name	FARMERS NATIONAL BANC CORP /OH/
Entity Central Index Key	0000709337
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Document Type	10-Q
Document Period End Date	Sep. 30, 2019
Amendment Flag	false
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		27,668,632
Entity Shell Company	false
Title of 12(b) Security	Common Stock, No Par Value
Trading Symbol	FMNB
Security Exchange Name	NASDAQ
Entity File Number	001-35296
Entity Incorporation, State or Country Code	OH
Entity Tax Identification Number	34-1371693
Entity Address, Address Line One	20 South Broad Street
Entity Address, City or Town	Canfield
Entity Address, State or Province	OH
Entity Address, Postal Zip Code	44406
City Area Code	330
Local Phone Number	533-3341
Document Quarterly Report	true
Document Transition Report	false