Parent Company Only Condensed Financial Information - Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income	$ 11,357	$ 10,869	$ 11,011	$ 8,639	$ 9,675	$ 9,154	$ 8,543	$ 8,388	$ 41,876	$ 35,760	$ 32,569
NET CASH FROM OPERATING ACTIVITIES									50,008	38,761	38,764
Cash flows from investing activities:
NET CASH FROM INVESTING ACTIVITIES									(160,829)	(93,146)	(179,321)
Cash flows from financing activities:
Repurchase of common shares									(14,238)	(2,842)	0
Cash dividends paid									(12,654)	(10,539)	(8,316)
NET CASH FROM FINANCING ACTIVITIES									294,682	67,219	140,869
Farmers National Banc Corp
Cash flows from operating activities:
Net income									41,876	35,760	32,569
Dividends in excess of net income (Equity in undistributed net income of subsidiary)									(12,461)	(1,009)	(23,737)
Other									1,167	(30)	1
NET CASH FROM OPERATING ACTIVITIES									30,582	34,721	8,833
Cash flows from investing activities:
Net cash paid in business combinations									(20,423)	0	0
NET CASH FROM INVESTING ACTIVITIES									(20,423)	0	0
Cash flows from financing activities:
Repurchase of common shares									(14,238)	(2,842)	0
Cash dividends paid									(12,654)	(10,539)	(8,316)
NET CASH FROM FINANCING ACTIVITIES									(26,892)	(13,381)	(8,316)
NET CHANGE IN CASH AND CASH EQUIVALENTS									(16,733)	21,340	517
Beginning cash and cash equivalents				$ 25,471				$ 4,131	25,471	4,131	3,614
Ending cash and cash equivalents	$ 8,738				$ 25,471				$ 8,738	$ 25,471	$ 4,131