Minimum Capital Requirements - Schedule of Actual and Required Capital Amounts and Ratios (Details) $ in Thousands	Mar. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
FB Financial Corporation
Total Capital (to risk-weighted assets)
Actual, amount	$ 1,739,703	$ 1,721,941
Actual, ratio	0.152	0.152
Minimum requirement for capital adequacy with capital buffer, amount	$ 1,202,607	$ 1,187,163
Minimum requirement for capital adequacy with capital buffer, ratio	0.105	0.105
Tier 1 Capital (to risk-weighted assets)
Actual, amount	$ 1,496,448	$ 1,480,722
Actual, ratio	0.131	0.131
Minimum requirement for capital adequacy with capital buffer, amount	$ 973,539	$ 961,037
Minimum requirement for capital adequacy with capital buffer, ratio	0.085	0.085
Common Equity Tier 1 Capital (to risk-weighted assets)
Actual, amount	$ 1,466,448	$ 1,450,722
Actual ratio	0.128	0.128
Minimum requirement for capital adequacy with capital buffer, amount	$ 801,738	$ 791,442
Minimum requirement for capital adequacy with capital buffer, ratio	0.070	0.070
Tier 1 Capital (to average assets)
Actual, amount	$ 1,496,448	$ 1,480,722
Actual, ratio	0.114	0.113
Minimum requirement for capital adequacy with capital buffer, amount	$ 523,095	$ 522,557
Minimum requirement for capital adequacy with capital buffer, ratio	0.040	0.040
FirstBank
Total Capital (to risk-weighted assets)
Actual, amount	$ 1,677,611	$ 1,650,305
Actual, ratio	0.148	0.147
Minimum requirement for capital adequacy with capital buffer, amount	$ 1,191,615	$ 1,175,095
Minimum requirement for capital adequacy with capital buffer, ratio	0.105	0.105
To qualify as well-capitalized under prompt corrective action provision, amount	$ 1,134,871	$ 1,119,138
To qualify as well-capitalized under prompt corrective action provision, ratio	0.100	0.100
Tier 1 Capital (to risk-weighted assets)
Actual, amount	$ 1,435,648	$ 1,410,505
Actual, ratio	0.127	0.126
Minimum requirement for capital adequacy with capital buffer, amount	$ 964,640	$ 951,267
Minimum requirement for capital adequacy with capital buffer, ratio	0.085	0.085
To qualify as well-capitalized under prompt corrective action provision, amount	$ 907,897	$ 895,310
To qualify as well-capitalized under prompt corrective action provision, ratio	0.080	0.080
Common Equity Tier 1 Capital (to risk-weighted assets)
Actual, amount	$ 1,435,648	$ 1,410,505
Actual ratio	0.127	0.126
Minimum requirement for capital adequacy with capital buffer, amount	$ 794,410	$ 783,397
Minimum requirement for capital adequacy with capital buffer, ratio	0.070	0.070
To qualify as well-capitalized under prompt corrective action provision, amount	$ 737,666	$ 727,440
To qualify as well-capitalized under prompt corrective action provision, ratio	0.065	0.065
Tier 1 Capital (to average assets)
Actual, amount	$ 1,435,648	$ 1,410,505
Actual, ratio	0.110	0.108
Minimum requirement for capital adequacy with capital buffer, amount	$ 521,397	$ 521,538
Minimum requirement for capital adequacy with capital buffer, ratio	0.040	0.040
To qualify as well-capitalized under prompt corrective action provision, amount	$ 651,746	$ 651,923
To qualify as well-capitalized under prompt corrective action provision, ratio	0.050	0.050