Parent Company Only Condensed Financial Information (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Assets:                        
Securities available for sale $ 394,312       $ 389,829       $ 394,312 $ 389,829    
Other 33,585       20,750       33,585 20,750    
TOTAL ASSETS 1,869,902       1,136,967       1,869,902 1,136,967    
Liabilities:                        
Other liabilities 14,823       10,187       14,823 10,187    
TOTAL LIABILITIES 1,671,855       1,013,407       1,671,855 1,013,407    
TOTAL STOCKHOLDERS' EQUITY 198,047       123,560       198,047 123,560 $ 113,007  
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY 1,869,902       1,136,967       1,869,902 1,136,967    
Income:                        
Dividends from subsidiaries                 287 190 196  
INCOME TAXES 848 $ 625 $ 409 $ 617 597 $ 688 $ 720 $ 627 2,499 2,632 1,683  
NET INCOME 3,175 1,857 812 2,211 2,147 2,276 2,351 2,191 8,055 8,965 7,780  
Cash flows from operating activities:                        
Net income 3,175 $ 1,857 $ 812 $ 2,211 2,147 $ 2,276 $ 2,351 $ 2,191 8,055 8,965 7,780  
Security gains         (372)       (94) (457) (863)  
Impairment of securities                 0 0 3  
Cash flows from investing activities:                        
Proceeds from maturities of available for sale securities                 63,243 49,401 75,015  
Net cash received (paid) in business combinations                 29,749 0 (2,111)  
NET CASH FROM INVESTING ACTIVITIES                 (23,113) 6,184 (27,981)  
CASH FLOWS FROM FINANCING ACTIVITIES                        
Proceeds from reissuance of treasury shares                 0 32 0  
Cash dividends paid                 (2,683) (2,236) (2,248)  
NET CHANGE IN CASH AND CASH EQUIVALENTS                 28,586 (85) (10,246)  
Farmers National Banc Corp                        
Assets:                        
Cash 1,357       1,564       1,357 1,564    
Securities available for sale 231       172       231 172    
Other 1,319       916       1,319 916    
TOTAL ASSETS 200,739       124,075       200,739 124,075    
Liabilities:                        
Other liabilities 241       163       241 163    
Note payable 350       350       350 350    
Subordinate debt 2,099       0       2,099 0    
Other accounts payable 2       2       2 2    
TOTAL LIABILITIES 2,692       515       2,692 515    
TOTAL STOCKHOLDERS' EQUITY 198,047       123,560       198,047 123,560    
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY 200,739       124,075       200,739 124,075    
Income:                        
Interest and dividends on securities                 2 1 2  
Security gains/(losses)                 0 0 21  
Other income                 0 764 0  
TOTAL INCOME                 24,896 6,778 5,336  
Interest on borrowings                 (35) (15) (16)  
Other expenses                 (4,817) (1,492) (1,659)  
Income before income tax benefit and undistributed subsidiary income                 20,044 5,271 3,661  
INCOME TAXES                 991 512 562  
Equity in undistributed net income of subsidiaries (dividends in excess of net income)                 (12,980) 3,182 3,557  
NET INCOME                 8,055 8,965 7,780  
Cash flows from operating activities:                        
Net income                 8,055 8,965 7,780  
Security gains                 0 0 (24)  
Impairment of securities                 0 0 3  
Dividends in excess of net income (Equity in undistributed net income of subsidiary)                 12,980 (3,182) (3,557)  
Other                 (269) (982) (270)  
NET CASH FROM OPERATING ACTIVITIES                 20,766 4,801 3,932  
Cash flows from investing activities:                        
Proceeds from maturities of available for sale securities                 0 0 56  
Net cash received (paid) in business combinations                 0 0 (2,111)  
Net cash paid in business combinations                 (18,077) 0 0  
NET CASH FROM INVESTING ACTIVITIES                 (18,077) 0 (2,055)  
CASH FLOWS FROM FINANCING ACTIVITIES                        
Proceeds from reissuance of treasury shares                 0 32 0  
Purchase of treasury shares                 (213) (2,882) (1,606)  
Cash dividends paid                 (2,683) (2,236) (2,248)  
NET CASH FROM FINANCING ACTIVITIES                 (2,896) (5,086) (3,854)  
NET CHANGE IN CASH AND CASH EQUIVALENTS                 (207) (285) (1,977)  
Beginning cash and cash equivalents 1,357       1,564       1,357 1,564 1,849 $ 3,826
Farmers National Banc Corp | National Associates Incorporated                        
Assets:                        
Investment in subsidiaries 3,391       3,604       3,391 3,604    
Income:                        
Dividends from subsidiaries                 400 0 0  
Equity in undistributed net income of subsidiaries (dividends in excess of net income)                 (214) (671) (156)  
Cash flows from operating activities:                        
Dividends in excess of net income (Equity in undistributed net income of subsidiary)                 214 671 156  
Farmers National Banc Corp | Bank Segment                        
Assets:                        
Investment in subsidiaries 184,253       107,704       184,253 107,704    
Income:                        
Dividends from subsidiaries                 23,744 4,013 4,333  
Equity in undistributed net income of subsidiaries (dividends in excess of net income)                 (12,837) 4,987 4,162  
Cash flows from operating activities:                        
Dividends in excess of net income (Equity in undistributed net income of subsidiary)                 12,837 (4,987) (4,162)  
Farmers National Banc Corp | Trust Segment                        
Assets:                        
Investment in subsidiaries $ 10,188       $ 10,115       10,188 10,115    
Income:                        
Dividends from subsidiaries                 750 2,000 980  
Equity in undistributed net income of subsidiaries (dividends in excess of net income)                 71 (1,134) (449)  
Cash flows from operating activities:                        
Dividends in excess of net income (Equity in undistributed net income of subsidiary)                 $ (71) $ 1,134 $ 449