Minimum capital requirements - Schedule of Actual and Required Capital Amounts and Ratios (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
FB Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to risk-weighted assets), Actual Amount	$ 614,708	$ 582,945
Total Capital (to risk-weighted assets), Actual Ratio	12.00%	13.00%
Total Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 409,805	$ 358,735
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	$ 537,870	$ 442,814
Total Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	10.50%	9.90%
Tier 1 Capital (to risk-weighted assets), Actual Amount	$ 583,244	$ 554,013
Tier 1 Capital (to risk-weighted assets), Actual Ratio	11.30%	12.40%
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 309,687	$ 268,071
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	$ 438,723	$ 351,843
Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	8.50%	7.90%
Tier 1 Capital (to average assets), Actual Amount	$ 583,244	$ 554,013
Tier 1 Capital (to average assets), Actual Ratio	10.10%	11.40%
Tier 1 Capital (to average assets), For capital adequacy purposes, Amount	$ 230,988	$ 194,391
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	$ 553,244	$ 524,013
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Ratio	10.80%	11.70%
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 230,518	$ 201,543
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	$ 358,584	$ 285,520
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	7.00%	6.40%
FirstBank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to risk-weighted assets), Actual Amount	$ 601,769	$ 561,327
Total Capital (to risk-weighted assets), Actual Ratio	11.70%	12.50%
Total Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 411,466	$ 359,249
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	$ 540,049	$ 443,448
Total Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	10.50%	9.90%
Total Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Amount	$ 514,332	$ 449,062
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	$ 570,305	$ 532,395
Tier 1 Capital (to risk-weighted assets), Actual Ratio	11.10%	11.90%
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 308,273	$ 268,434
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	$ 436,720	$ 352,320
Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	8.50%	7.90%
Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions Amount	$ 411,031	$ 357,913
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	$ 570,305	$ 532,395
Tier 1 Capital (to average assets), Actual Ratio	9.80%	10.90%
Tier 1 Capital (to average assets), For capital adequacy purposes, Amount	$ 232,778	$ 195,374
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	$ 290,972	$ 244,218
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	$ 570,305	$ 532,395
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Ratio	11.10%	11.90%
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 231,205	$ 201,326
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	$ 359,652	$ 285,212
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	7.00%	6.40%
Common Equity Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Amount	$ 333,962	$ 290,804
Common Equity Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Ratio	6.50%	6.50%