Basis of presentation - Schedule of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Basic earnings per common share calculation:
Net (loss) income applicable to FB Financial Corporation	$ (5,599)	$ 23,966	$ 18,019	$ 62,242
Dividends paid on and undistributed earnings allocated to participating securities	0	(128)	0	(333)
(Loss) earnings available to common shareholders	$ (5,599)	$ 23,838	$ 18,019	$ 61,909
Weighted average basic shares outstanding (in shares)	40,154,841	30,899,583	34,404,064	30,849,035
Basic (loss) earnings per common share (in dollars per share)	$ (0.14)	$ 0.77	$ 0.52	$ 2.01
Diluted earnings per common share:
(Loss) earnings available to common shareholders	$ (5,599)	$ 23,838	$ 18,019	$ 61,909
Weighted average basic shares outstanding (in shares)	40,154,841	30,899,583	34,404,064	30,849,035
Weighted average diluted shares contingently issuable (in shares)	482,904	525,990	436,228	529,751
Weighted average diluted shares outstanding (in shares)	40,637,745	31,425,573	34,840,292	31,378,786
Diluted (loss) earnings per common share (in dollars per share)	$ (0.14)	$ 0.76	$ 0.52	$ 1.97
Restricted stock units outstanding considered to be antidilutive (in shares)	332,347		536,908