| Cover | Apr. 22, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Apr. 22, 2021 |
| Entity File Number | 001-37621 |
| Entity Registrant Name | FIRST GUARANTY BANCSHARES, INC. |
| Entity Central Index Key | 0001408534 |
| Entity Tax Identification Number | 26-0513559 |
| Entity Incorporation, State or Country Code | LA |
| Entity Address, Address Line One | 400 East Thomas Street |
| Entity Address, City or Town | Hammond |
| Entity Address, State or Province | LA |
| Entity Address, Postal Zip Code | 70401 |
| City Area Code | (985) |
| Local Phone Number | 345-7685 |
| 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 | FGBI |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Information, Former Legal or Registered Name | Not Applicable |
