Minimum Capital Requirements - Schedule of Actual and Required Capital Amounts and Ratios (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
FB Financial Corporation
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total Capital (to risk-weighted assets), Actual Amount	$ 496,422	$ 338,893
Total Capital (to risk-weighted assets), Actual Ratio	12.01%	13.03%
Total Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 330,672	$ 208,069
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	$ 382,340	$ 224,325
Total Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	9.25%	8.63%
Tier 1 Capital (to risk-weighted assets), Actual Amount	$ 472,381	$ 317,146
Tier 1 Capital (to risk-weighted assets), Actual Ratio	11.43%	12.19%
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 247,969	$ 156,101
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	$ 299,629	$ 172,362
Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	7.25%	6.63%
Tier 1 Capital (to average assets), Actual Amount	$ 472,381	$ 317,146
Tier 1 Capital (to average assets), Actual Ratio	10.46%	10.05%
Tier 1 Capital (to average assets), For capital adequacy purposes, Amount	$ 180,643	$ 126,227
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	$ 442,381	$ 287,146
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Ratio	10.71%	11.04%
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 185,874	$ 117,043
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	$ 237,506	$ 133,299
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	5.75%	5.13%
FirstBank
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total Capital (to risk-weighted assets), Actual Amount	$ 466,102	$ 304,018
Total Capital (to risk-weighted assets), Actual Ratio	11.30%	11.72%
Total Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 329,984	$ 207,521
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	$ 381,544	$ 223,733
Total Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	9.25%	8.63%
Total Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Amount	$ 412,480	$ 259,401
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	$ 442,061	$ 282,271
Tier 1 Capital (to risk-weighted assets), Actual Ratio	10.72%	10.88%
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 247,422	$ 155,664
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	$ 298,968	$ 171,879
Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	7.25%	6.63%
Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions Amount	$ 247,422	$ 155,664
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	$ 442,061	$ 282,271
Tier 1 Capital (to average assets), Actual Ratio	9.77%	8.95%
Tier 1 Capital (to average assets), For capital adequacy purposes, Amount	$ 180,987	$ 126,155
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	$ 226,234	$ 157,693
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	$ 442,061	$ 282,271
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Ratio	10.72%	10.88%
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 185,567	$ 116,748
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	$ 237,113	$ 132,963
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	5.75%	5.13%
Common Equity Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Amount	$ 268,041	$ 168,636
Common Equity Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Ratio	6.50%	6.50%