Parent Company Only Condensed Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets:
Securities available for sale	$ 393,331								$ 369,995				$ 393,331	$ 369,995
Other	41,254								38,775				41,254	38,775
TOTAL ASSETS	2,159,069								1,966,113				2,159,069	1,966,113
Liabilities:
Other liabilities	15,717								14,645				15,717	14,645
TOTAL LIABILITIES	1,916,995								1,752,897				1,916,995	1,752,897
TOTAL STOCKHOLDERS' EQUITY	242,074								213,216				242,074	213,216	$ 198,047
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	2,159,069								1,966,113				2,159,069	1,966,113
Income:
Dividends from subsidiaries													537	515	287
Income tax benefit	(4,084)	[1]	$ (2,009)	[1]	$ (2,004)	[1]	$ (1,972)	[1]	(2,014)	$ (1,967)	$ (1,833)	$ (1,671)	(10,069)	(7,485)	(2,499)
NET INCOME	5,235		5,983		5,710		5,783		5,363	5,376	5,020	4,798	22,711	20,557	8,055
Cash flows from operating activities:
Net income	5,235		$ 5,983		$ 5,710		5,783		5,363	$ 5,376	$ 5,020	4,798	22,711	20,557	8,055
Security gains													(4)	(73)	(94)
Cash flows from investing activities:
Proceeds from maturities of available for sale securities													43,104	59,904	63,243
Cash flows from financing activities:
Repurchase of common shares													0	(168)	(213)
Cash dividends paid													(6,012)	(4,326)	(2,683)
NET CHANGE IN CASH AND CASH EQUIVALENTS													15,836	(14,236)	28,586
Farmers National Banc Corp
Assets:
Cash	3,614								2,620				3,614	2,620
Securities available for sale	258								234				258	234
Other	1,442								1,284				1,442	1,284
TOTAL ASSETS	244,885								215,785				244,885	215,785
Liabilities:
Other liabilities	250								57				250	57
Note payable	350								350				350	350
Subordinate debt	2,209								2,160				2,209	2,160
Other accounts payable	2								2				2	2
TOTAL LIABILITIES	2,811								2,569				2,811	2,569
TOTAL STOCKHOLDERS' EQUITY	242,074								213,216				242,074	213,216
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	244,885								215,785				244,885	215,785
Income:
Interest and dividends on securities													5	5	2
Security gains/(losses)													0	(19)	0
Other income													0	28	0
TOTAL INCOME													9,603	7,470	24,896
Interest on borrowings													(103)	(96)	(35)
Other expenses													(2,294)	(1,997)	(4,817)
Income before income tax benefit and undistributed subsidiary income													7,206	5,377	20,044
Income tax benefit													876	670	991
Equity in undistributed net income of subsidiaries (dividends in excess of net income)													14,629	14,510	(12,980)
NET INCOME													22,711	20,557	8,055
Cash flows from operating activities:
Net income													22,711	20,557	8,055
Security gains													0	19	0
Dividends in excess of net income (Equity in undistributed net income of subsidiary)													(14,629)	(14,510)	12,980
Other													78	(368)	(269)
NET CASH FROM OPERATING ACTIVITIES													8,160	5,698	20,766
Cash flows from investing activities:
Proceeds from maturities of available for sale securities													0	59	0
Net cash paid in business combinations													(1,154)	0	(18,077)
NET CASH FROM INVESTING ACTIVITIES													(1,154)	59	(18,077)
Cash flows from financing activities:
Proceeds from reissuance of treasury shares													0	0	0
Repurchase of common shares													0	(168)	(213)
Cash dividends paid													(6,012)	(4,326)	(2,683)
NET CASH FROM FINANCING ACTIVITIES													(6,012)	(4,494)	(2,896)
NET CHANGE IN CASH AND CASH EQUIVALENTS													994	1,263	(207)
Beginning cash and cash equivalents							$ 2,620					$ 1,357	2,620	1,357	1,564
Ending cash and cash equivalents	3,614								2,620				3,614	2,620	1,357
Farmers National Banc Corp | National Associates Incorporated
Assets:
Investment in subsidiaries	2,649								2,787				2,649	2,787
Income:
Dividends from subsidiaries													400	800	400
Equity in undistributed net income of subsidiaries (dividends in excess of net income)													(138)	(604)	(214)
Cash flows from operating activities:
Dividends in excess of net income (Equity in undistributed net income of subsidiary)													138	604	214
Farmers National Banc Corp | Bank Segment
Assets:
Investment in subsidiaries	224,843								198,030				224,843	198,030
Income:
Dividends from subsidiaries													8,373	5,836	23,744
Equity in undistributed net income of subsidiaries (dividends in excess of net income)													13,529	14,688	(12,837)
Cash flows from operating activities:
Dividends in excess of net income (Equity in undistributed net income of subsidiary)													(13,529)	(14,688)	12,837
Farmers National Banc Corp | Trust Segment
Assets:
Investment in subsidiaries	10,495								10,184				10,495	10,184
Income:
Dividends from subsidiaries													825	820	750
Equity in undistributed net income of subsidiaries (dividends in excess of net income)													299	31	71
Cash flows from operating activities:
Dividends in excess of net income (Equity in undistributed net income of subsidiary)													(299)	(31)	(71)
Farmers National Banc Corp | Captive
Assets:
Investment in subsidiaries	$ 1,584								$ 646				1,584	646
Income:
Equity in undistributed net income of subsidiaries (dividends in excess of net income)													939	395	0
Cash flows from operating activities:
Dividends in excess of net income (Equity in undistributed net income of subsidiary)													$ (939)	$ (395)	$ 0

[1] Income tax expense for the fourth quarter was elevated due to the additional $1.8 million of tax expense from the re-measurement of the Company’s net deferred tax asset.