Minimum capital requirements - Schedule of Actual and Required Capital Amounts and Ratios (Details) $ in Thousands	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
FB Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to risk-weighted assets), Actual Amount	$ 735,555	$ 633,549
Total Capital (to risk-weighted assets), Actual Ratio	0.134	0.122
Total Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 439,137	$ 415,442
Total Capital (to risk-weighted assets), For capital adequacy purposes, Ratio	0.080	0.080
Total Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Amount	$ 576,368	$ 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	$ 666,090	$ 602,410
Tier 1 Capital (to risk-weighted assets), Actual Ratio	0.121	0.116
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 330,293	$ 311,591
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Ratio	0.060	0.060
Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Amount	$ 467,914	$ 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	$ 666,090	$ 602,410
Tier 1 Capital (to average assets), Actual Ratio	0.097	0.101
Tier 1 Capital (to average assets), For capital adequacy purposes, Amount	$ 274,676	$ 238,578
Tier 1 Capital (to average assets), For capital adequacy purposes, Ratio	0.040	0.040
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Amount	$ 636,090	$ 572,410
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Ratio	11.60%	11.10%
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 246,759	$ 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	$ 383,847	$ 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	$ 742,321	$ 623,432
Total Capital (to risk-weighted assets), Actual Ratio	0.135	0.121
Total Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 439,894	$ 412,186
Total Capital (to risk-weighted assets), For capital adequacy purposes, Ratio	0.080	0.080
Total Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Amount	$ 577,361	$ 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	$ 549,867	$ 515,233
Total Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions, Ratio	0.100	0.100
Tier 1 Capital (to risk-weighted assets), Actual Amount	$ 672,856	$ 592,293
Tier 1 Capital (to risk-weighted assets), Actual Ratio	0.123	0.115
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 328,222	$ 309,022
Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Ratio	0.060	0.060
Tier 1 Capital (to risk-weighted assets), Minimum Capital adequacy with capital buffer, Amount	$ 464,982	$ 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	$ 437,630	$ 412,030
Tier 1 Capital (to risk-weighted assets), To be well capitalized under prompt corrective action provisions Ratio	0.080	0.080
Tier 1 Capital (to average assets), Actual Amount	$ 672,856	$ 592,293
Tier 1 Capital (to average assets), Actual Ratio	0.097	0.099
Tier 1 Capital (to average assets), For capital adequacy purposes, Amount	$ 277,466	$ 239,310
Tier 1 Capital (to average assets), For capital adequacy purposes, Ratio	0.040	0.040
Tier 1 Capital (to average assets), To be well capitalized under prompt corrective action provisions, Amount	$ 346,833	$ 299,138
Tier 1 Capital (to average assets), To be well capitalized under prompt corrective action provisions, Ratio	0.050	0.050
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Amount	$ 672,856	$ 592,293
Common Equity Tier 1 Capital (to risk-weighted assets), Actual Ratio	12.30%	11.50%
Common Equity Tier 1 Capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 246,167	$ 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	$ 382,926	$ 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	$ 355,574	$ 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%