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Parent Company Only Condensed Financial Information - Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income	$ 9,675	$ 9,154	$ 8,543	$ 8,388	$ 8,686	$ 8,084	$ 8,073	$ 7,726	$ 35,760	$ 32,569	$ 22,711
NET CASH FROM OPERATING ACTIVITIES									38,761	39,989	30,566
Cash flows from investing activities:
NET CASH FROM INVESTING ACTIVITIES									(93,146)	(179,321)	(137,109)
Cash flows from financing activities:
Repurchase of common shares									(2,842)	(1,225)	0
Cash dividends paid									(10,539)	(8,316)	(6,012)
NET CASH FROM FINANCING ACTIVITIES									67,219	139,644	122,379
Farmers National Banc Corp
Cash flows from operating activities:
Net income									35,760	32,569	22,711
Dividends in excess of net income (Equity in undistributed net income of subsidiary)									(1,009)	(23,737)	(14,629)
Other									(30)	1	78
NET CASH FROM OPERATING ACTIVITIES									34,721	8,833	8,160
Cash flows from investing activities:
Net cash paid in business combinations									0	0	(1,154)
NET CASH FROM INVESTING ACTIVITIES									0	0	(1,154)
Cash flows from financing activities:
Repurchase of common shares									(2,842)	0	0
Cash dividends paid									(10,539)	(8,316)	(6,012)
NET CASH FROM FINANCING ACTIVITIES									(13,381)	(8,316)	(6,012)
NET CHANGE IN CASH AND CASH EQUIVALENTS									21,340	517	994
Beginning cash and cash equivalents				$ 4,131				$ 3,614	4,131	3,614	2,620
Ending cash and cash equivalents	$ 25,471				$ 4,131				$ 25,471	$ 4,131	$ 3,614