Minimum Capital Requirements - Schedule of Actual and Required Capital Amounts and Ratios (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
FB Financial Corporation
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total Capital (to risk-weighted assets), Actual Amount	$ 524,655	$ 496,422
Total Capital (to risk-weighted assets), Actual Ratio	11.90%	12.00%
Total Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 352,709	$ 330,672
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	$ 435,375	$ 382,340
Total Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	9.90%	9.30%
Tier 1 Capital (to risk-weighted assets), Actual Amount	$ 498,308	$ 472,381
Tier 1 Capital (to risk-weighted assets), Actual Ratio	11.30%	11.40%
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 264,588	$ 247,969
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	$ 347,272	$ 299,629
Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	7.90%	7.30%
Tier 1 Capital (to average assets), Actual Amount	$ 498,308	$ 472,381
Tier 1 Capital (to average assets), Actual Ratio	10.90%	10.50%
Tier 1 Capital (to average assets), For capital adequacy purposes, Amount	$ 182,865	$ 180,643
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	$ 468,308	$ 442,381
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Ratio	10.60%	10.70%
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 198,810	$ 185,874
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	$ 281,648	$ 237,506
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	6.40%	5.80%
FirstBank
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total Capital (to risk-weighted assets), Actual Amount	$ 497,123	$ 466,102
Total Capital (to risk-weighted assets), Actual Ratio	11.30%	11.30%
Total Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 351,945	$ 329,984
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	$ 434,433	$ 381,544
Total Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	9.90%	9.30%
Total Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Amount	$ 439,932	$ 412,480
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	$ 470,776	$ 442,061
Tier 1 Capital (to risk-weighted assets), Actual Ratio	10.70%	10.70%
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 263,987	$ 247,422
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	$ 346,482	$ 298,968
Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	7.90%	7.30%
Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions Amount	$ 263,987	$ 247,422
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	$ 470,776	$ 442,061
Tier 1 Capital (to average assets), Actual Ratio	10.20%	9.80%
Tier 1 Capital (to average assets), For capital adequacy purposes, Amount	$ 184,618	$ 180,987
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	$ 230,773	$ 226,234
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	$ 470,776	$ 442,061
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Ratio	10.70%	10.70%
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 197,990	$ 185,567
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	$ 280,486	$ 237,113
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	6.40%	5.80%
Common Equity Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Amount	$ 285,985	$ 268,041
Common Equity Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Ratio	6.50%	6.50%