Minimum capital requirements - Schedule of Actual and Required Capital Amounts and Ratios (Details) - USD ($) $ in Thousands	Dec. 31, 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	$ 582,945	$ 496,422
Total Capital (to risk-weighted assets), Actual Ratio	13.00%	12.00%
Total Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 358,735	$ 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	$ 442,814	$ 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	$ 554,013	$ 472,381
Tier 1 Capital (to risk-weighted assets), Actual Ratio	12.40%	11.40%
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 268,071	$ 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	$ 351,843	$ 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	$ 554,013	$ 472,381
Tier 1 Capital (to average assets), Actual Ratio	11.40%	10.50%
Tier 1 Capital (to average assets), For capital adequacy purposes, Amount	$ 194,391	$ 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	$ 524,013	$ 442,381
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	$ 201,543	$ 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	$ 285,520	$ 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	$ 561,327	$ 466,102
Total Capital (to risk-weighted assets), Actual Ratio	12.50%	11.30%
Total Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 359,249	$ 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	$ 443,448	$ 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	$ 449,062	$ 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	$ 532,395	$ 442,061
Tier 1 Capital (to risk-weighted assets), Actual Ratio	11.90%	10.70%
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 268,434	$ 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	$ 352,320	$ 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	$ 357,913	$ 329,896
Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions Ratio	8.00%	8.00%
Tier 1 Capital (to average assets), Actual Amount	$ 532,395	$ 442,061
Tier 1 Capital (to average assets), Actual Ratio	10.90%	9.80%
Tier 1 Capital (to average assets), For capital adequacy purposes, Amount	$ 195,374	$ 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	$ 244,218	$ 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	$ 532,395	$ 442,061
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Ratio	11.90%	10.70%
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 201,326	$ 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	$ 285,212	$ 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	$ 290,804	$ 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%