Minimum Capital Requirements - Schedule of Actual and Required Capital Amounts and Ratios (Details) - USD ($) $ in Thousands	Sep. 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	$ 567,909	$ 496,422
Total Capital (to risk-weighted assets), Actual Ratio	12.80%	12.00%
Total Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 354,943	$ 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	$ 432,587	$ 382,340
Total Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	9.80%	9.30%
Tier 1 Capital (to risk-weighted assets), Actual Amount	$ 540,301	$ 472,381
Tier 1 Capital (to risk-weighted assets), Actual Ratio	12.20%	11.40%
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 265,722	$ 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	$ 343,224	$ 299,629
Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	7.80%	7.30%
Tier 1 Capital (to average assets), Actual Amount	$ 540,301	$ 472,381
Tier 1 Capital (to average assets), Actual Ratio	11.30%	10.50%
Tier 1 Capital (to average assets), For capital adequacy purposes, Amount	$ 191,257	$ 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	$ 510,301	$ 442,381
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Ratio	11.50%	10.70%
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 199,683	$ 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	$ 277,338	$ 237,506
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	6.30%	5.80%
FirstBank
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total Capital (to risk-weighted assets), Actual Amount	$ 542,824	$ 466,102
Total Capital (to risk-weighted assets), Actual Ratio	12.30%	11.30%
Total Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 353,056	$ 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	$ 430,287	$ 381,544
Total Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	9.80%	9.30%
Total Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Amount	$ 441,320	$ 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	$ 515,216	$ 442,061
Tier 1 Capital (to risk-weighted assets), Actual Ratio	11.70%	10.70%
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 264,213	$ 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	$ 341,276	$ 298,968
Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	7.80%	7.30%
Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions Amount	$ 264,213	$ 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	$ 515,216	$ 442,061
Tier 1 Capital (to average assets), Actual Ratio	10.70%	9.80%
Tier 1 Capital (to average assets), For capital adequacy purposes, Amount	$ 192,604	$ 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	$ 240,755	$ 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	$ 515,216	$ 442,061
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Ratio	11.70%	10.70%
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 198,160	$ 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	$ 275,222	$ 237,113
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	6.30%	5.80%
Common Equity Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Amount	$ 286,231	$ 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%