Minimum Capital Requirements - Schedule of Actual and Required Capital Amounts and Ratios (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
FB Financial Corporation
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total Capital (to risk-weighted assets), Actual Amount	$ 328,387	$ 246,168
Total Capital (to risk-weighted assets), Actual Ratio	13.32%	11.15%
Total Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 197,229	$ 176,623
Total Capital (to risk-weighted assets), For capital adequacy purposes, Ratio	8.00%	8.00%
Total Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Amount	$ 212,638
Total Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	8.625%
Tier 1 Capital (to risk-weighted assets), Actual Amount	$ 305,097	$ 211,633
Tier 1 Capital (to risk-weighted assets), Actual Ratio	12.37%	9.58%
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 147,986	$ 132,547
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Ratio	6.00%	6.00%
Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Amount	$ 163,401
Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	6.625%
Tier 1 Capital (to average assets), Actual Amount	$ 305,097	$ 211,633
Tier 1 Capital (to average assets), Actual Ratio	10.32%	7.64%
Tier 1 Capital (to average assets), For capital adequacy purposes, Amount	$ 118,255	$ 110,803
Tier 1 Capital (to average assets), For capital adequacy purposes, Ratio	4.00%	4.00%
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Amount	$ 275,097	$ 181,633
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Ratio	11.16%	8.23%
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 110,926	$ 99,313
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Ratio	4.50%	4.50%
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Amount	$ 126,333
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	5.125%
FirstBank
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total Capital (to risk-weighted assets), Actual Amount	$ 292,830	$ 242,240
Total Capital (to risk-weighted assets), Actual Ratio	11.91%	11.02%
Total Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 196,695	$ 175,855
Total Capital (to risk-weighted assets), For capital adequacy purposes, Ratio	8.00%	8.00%
Total Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Amount	$ 212,062
Total Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	8.625%
Total Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Amount	$ 245,869	$ 219,819
Total Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Ratio	10.00%	10.00%
Tier 1 Capital (to risk-weighted assets), Actual Amount	$ 269,540	$ 211,780
Tier 1 Capital (to risk-weighted assets), Actual Ratio	10.96%	9.63%
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 147,558	$ 131,950
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Ratio	6.00%	6.00%
Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Amount	$ 162,929
Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	6.625%
Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions Amount	$ 147,558	$ 131,950
Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions Ratio	6.00%	6.00%
Tier 1 Capital (to average assets), Actual Amount	$ 269,540	$ 211,780
Tier 1 Capital (to average assets), Actual Ratio	9.12%	7.65%
Tier 1 Capital (to average assets), For capital adequacy purposes, Amount	$ 118,219	$ 110,735
Tier 1 Capital (to average assets), For capital adequacy purposes, Ratio	4.00%	4.00%
Tier 1 Capital (to average assets), To be well capitalized under prompt corrective action provisions, Amount	$ 147,774	$ 138,418
Tier 1 Capital (to average assets), To be well capitalized under prompt corrective action provisions, Ratio	5.00%	5.00%
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Amount	$ 269,540	$ 211,780
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Ratio	10.96%	9.63%
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 110,669	$ 98,963
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Ratio	4.50%	4.50%
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Amount	$ 126,039
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	5.125%
Common Equity Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Amount	$ 159,855	$ 142,946
Common Equity Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Ratio	6.50%	6.50%