Minimum capital requirements (Tables)
 12 Months Ended
Dec. 31, 2020
Broker-Dealer, Net Capital Requirement, SEC Regulation [Abstract]  
Schedule of Actual and Required Capital Amounts and Ratios
Actual and required capital amounts and ratios are included below for the periods presented.

 ActualMinimum Capital
adequacy with
capital buffer		To be well capitalized
under prompt corrective
action provisions
 AmountRatioAmountRatioAmountRatio
December 31, 2020      
Total Capital (to risk-weighted assets)      
FB Financial Corporation$1,358,897 15.0 %$952,736 10.5 %N/AN/A
FirstBank1,353,279 14.9 %951,327 10.5 %$906,026 10.0 %
Tier 1 Capital (to risk-weighted assets)
FB Financial Corporation$1,090,364 12.0 %$771,262 8.5 %N/AN/A
FirstBank1,142,548 12.6 %770,122 8.5 %$724,820 8.0 %
Tier 1 Capital (to average assets)
FB Financial Corporation$1,090,364 10.0 %$435,064 4.0 %N/AN/A
FirstBank1,142,548 10.5 %435,279 4.0 %$544,098 5.0 %
Common Equity Tier 1 Capital
(to risk-weighted assets)
FB Financial Corporation$1,060,364 11.7 %$635,157 7.0 %N/AN/A
FirstBank1,142,548 12.6 %634,218 7.0 %$588,917 6.5 %
 ActualMinimum Capital
adequacy with
capital buffer		To be well capitalized
under prompt corrective
action provisions
 AmountRatioAmountRatioAmountRatio
December 31, 2019      
Total Capital (to risk-weighted assets)      
FB Financial Corporation$633,549 12.2 %$545,268 10.5 %N/AN/A
FirstBank623,432 12.1 %540,995 10.5 %$515,233 10.0 %
Tier 1 Capital (to risk-weighted assets)
FB Financial Corporation$602,410 11.6 %$441,421 8.50 %N/AN/A
FirstBank592,293 11.5 %437,782 8.50 %$412,030 8.0 %
Tier 1 Capital (to average assets)
FB Financial Corporation$602,410 10.1 %$238,578 4.00 %N/AN/A
FirstBank592,293 9.9 %239,310 4.00 %$299,138 5.0 %
Common Equity Tier 1 Capital
(to risk-weighted assets)
FB Financial Corporation$572,410 11.1 %$360,979 7.0 %N/AN/A
FirstBank592,293 11.5 %360,526 7.0 %$334,774 6.5 %