Consolidated statements of changes in shareholders' equity - USD ($) $ in Thousands	Total	FNB Financial Corp.	Franklin Financial Network, Inc.	Period of adoption, adjustment	Period of adoption, adjusted balance	Common stock	Common stock FNB Financial Corp.	Common stock Franklin Financial Network, Inc.	Common stock Period of adoption, adjusted balance	Additional paid-in capital	Additional paid-in capital FNB Financial Corp.	Additional paid-in capital Franklin Financial Network, Inc.	Additional paid-in capital Period of adoption, adjusted balance	Retained earnings	Retained earnings Period of adoption, adjustment	Retained earnings Period of adoption, adjusted balance	Accumulated other comprehensive income, net	Accumulated other comprehensive income, net Period of adoption, adjusted balance	Total common shareholders' equity	Total common shareholders' equity FNB Financial Corp.	Total common shareholders' equity Franklin Financial Network, Inc.	Total common shareholders' equity Period of adoption, adjustment	Total common shareholders' equity Period of adoption, adjusted balance	Noncontrolling interests	Noncontrolling interests Franklin Financial Network, Inc.
Balance at Dec. 31, 2018	$ 671,857			$ (1,309)	$ 670,548	$ 30,725			$ 30,725	$ 424,146			$ 424,146	$ 221,213	$ (1,309)	$ 219,904	$ (4,227)	$ (4,227)	$ 671,857			$ (1,309)	$ 670,548
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	62,242													62,242					62,242
Other comprehensive income (loss), net of taxes	16,827																16,827		16,827
Stock based compensation expense	5,621					9				5,612									5,621
Restricted stock units vested and distributed, net of shares withheld	(3,552)					171				(3,723)									(3,552)
Shares issued under employee stock purchase program	804					23				781									804
Dividends declared	(7,655)													(7,655)					(7,655)
Balance at Sep. 30, 2019	744,835					30,928				426,816				274,491			12,600		744,835
Balance at Jun. 30, 2019	718,759					30,866				425,644				253,080			9,169		718,759
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	23,966													23,966					23,966
Other comprehensive income (loss), net of taxes	3,431																3,431		3,431
Stock based compensation expense	1,836					3				1,833									1,836
Restricted stock units vested and distributed, net of shares withheld	(1,042)					47				(1,089)									(1,042)
Shares issued under employee stock purchase program	440					12				428									440
Dividends declared	(2,555)													(2,555)					(2,555)
Balance at Sep. 30, 2019	744,835					30,928				426,816				274,491			12,600		744,835
Balance at Dec. 31, 2019	762,329			$ (25,018)	$ 737,311	31,034			$ 31,034	425,633			$ 425,633	293,524	$ (25,018)	$ 268,506	12,138	$ 12,138	762,329			$ (25,018)	$ 737,311
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	18,019													18,019					18,019
Other comprehensive income (loss), net of taxes	13,149																13,149		13,149
Common stock issued in connection with acquisition of FNB Financial Corp., net of registration costs		$ 34,847	$ 444,966				$ 955	$ 15,058			$ 33,892	$ 429,815								$ 34,847	$ 444,873				$ 93
Stock based compensation expense	7,253					17				7,236									7,253
Restricted stock units vested and distributed, net of shares withheld	(1,268)					98				(1,366)									(1,268)
Shares issued under employee stock purchase program	978					30				948									978
Dividends declared	(10,164)													(10,164)					(10,164)
Balance at Sep. 30, 2020	1,245,091					47,192				896,158				276,361			25,287		1,244,998					$ 93
Balance at Jun. 30, 2020	805,216					32,101				462,930				286,296			23,889		805,216
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	(5,599)													(5,599)					(5,599)
Other comprehensive income (loss), net of taxes	1,398																1,398		1,398
Common stock issued in connection with acquisition of FNB Financial Corp., net of registration costs			$ 444,966					$ 15,058				$ 429,815									$ 444,873				$ 93
Stock based compensation expense	3,020					6				3,014									3,020
Restricted stock units vested and distributed, net of shares withheld	(106)					9				(115)									(106)
Shares issued under employee stock purchase program	532					18				514									532
Dividends declared	(4,336)													(4,336)					(4,336)
Balance at Sep. 30, 2020	$ 1,245,091					$ 47,192				$ 896,158				$ 276,361			$ 25,287		$ 1,244,998					$ 93