Basis of presentation - Basic and Diluted Earnings Per Common Share Calculation (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Basic earnings per common share calculation:
Net income applicable to FB Financial Corporation	$ 190,285	$ 63,621	$ 83,814
Dividends paid on and undistributed earnings allocated to participating securities	0	0	(447)
Earnings available to common shareholders	$ 190,285	$ 63,621	$ 83,367
Weighted average basic shares outstanding (in shares)	47,431,102	37,621,720	30,870,474
Basic earnings (loss) per common share (in dollars per share)	$ 4.01	$ 1.69	$ 2.70
Diluted earnings per common share:
Earnings (loss) available to common shareholders	$ 190,285	$ 63,621	$ 83,367
Weighted average basic shares outstanding (in shares)	47,431,102	37,621,720	30,870,474
Weighted average diluted shares contingently issuable (in shares)	524,778	478,024	532,423
Weighted average diluted shares outstanding (in shares)	47,955,880	38,099,744	31,402,897
Diluted (loss) earnings per common share (in dollars per share)	$ 3.97	$ 1.67	$ 2.65
Restricted Stock Units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Restricted stock units outstanding considered to be antidilutive (in shares)	4,400	239,813