Minimum capital requirements - Schedule of Actual and Required Capital Amounts and Ratios (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
FB Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to risk-weighted assets), Actual Amount	$ 688,396	$ 633,549
Total Capital (to risk-weighted assets), Actual Ratio	12.50%	12.20%
Total Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 440,573	$ 415,442
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	$ 578,253	$ 545,268
Total Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	10.50%	10.50%
Tier 1 Capital (to risk-weighted assets), Actual Amount	$ 636,922	$ 602,410
Tier 1 Capital (to risk-weighted assets), Actual Ratio	11.60%	11.60%
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 329,442	$ 311,591
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	$ 466,710	$ 441,421
Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	8.50%	8.50%
Tier 1 Capital (to average assets), Actual Amount	$ 636,922	$ 602,410
Tier 1 Capital (to average assets), Actual Ratio	10.30%	10.10%
Tier 1 Capital (to average assets), For capital adequacy purposes, Amount	$ 247,348	$ 238,578
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	$ 606,922	$ 572,410
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Ratio	11.00%	11.10%
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 248,286	$ 232,058
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	$ 386,223	$ 360,979
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	7.00%	7.00%
FirstBank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to risk-weighted assets), Actual Amount	$ 696,625	$ 623,432
Total Capital (to risk-weighted assets), Actual Ratio	12.70%	12.10%
Total Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 438,819	$ 412,186
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	$ 575,950	$ 540,995
Total Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	10.50%	10.50%
Total Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Amount	$ 548,524	$ 515,233
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	$ 645,151	$ 592,293
Tier 1 Capital (to risk-weighted assets), Actual Ratio	11.70%	11.50%
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 330,847	$ 309,022
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	$ 468,699	$ 437,782
Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	8.50%	8.50%
Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions Amount	$ 441,129	$ 412,030
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	$ 645,151	$ 592,293
Tier 1 Capital (to average assets), Actual Ratio	10.40%	9.90%
Tier 1 Capital (to average assets), For capital adequacy purposes, Amount	$ 248,135	$ 239,310
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	$ 310,169	$ 299,138
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	$ 645,151	$ 592,293
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Ratio	11.70%	11.50%
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 248,135	$ 231,767
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	$ 385,988	$ 360,526
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Ratio	7.00%	7.00%
Common Equity Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Amount	$ 358,417	$ 334,774
Common Equity Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Ratio	6.50%	6.50%