| Cover | Nov. 25, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Nov. 25, 2020 |
| Entity Registrant Name | FB FINANCIAL CORPORATION |
| Entity Incorporation, State or Country Code | TN |
| Entity File Number | 001-37875 |
| Entity Tax Identification Number | 62-1216058 |
| Entity Address, Address Line One | 211 Commerce Street |
| Entity Address, Address Line Two | Suite 300 |
| Entity Address, City or Town | Nashville |
| Entity Address, State or Province | TN |
| Entity Address, Postal Zip Code | 37201 |
| City Area Code | 615 |
| Local Phone Number | 564-1212 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $1.00 par value |
| Trading Symbol | FBK |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
| Entity Central Index Key | 0001649749 |
| Amendment Flag | false |
