Cover
| Cover | May 20, 2021 | May 05, 2021 |
| Document Information [Line Items] | ||
| Pre-commencement Tender Offer | false | |
| Pre-commencement Issuer Tender Offer | false | |
| Entity Emerging Growth Company | false | |
| Entity Central Index Key | 0001408534 | |
| Document Type | 8-K | |
| Document Period End Date | Nov. 3, 2021 | |
| Entity Registrant Name | FIRST GUARANTY BANCSHARES, INC. | |
| Entity Incorporation, State or Country Code | LA | |
| Entity File Number | 001-37621 | |
| Entity Tax Identification Number | 26-0513559 | |
| 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 | 345-7685 | |
| City Area Code | (985) | |
| Written Communications | false | |
| Soliciting Material | false | |
| Amendment Flag | false | |
| Common Stock | ||
| Document Information [Line Items] | ||
| Title of 12(b) Security | Common Stock, $1 par value | |
| Trading Symbol | FGBI | |
| Security Exchange Name | NASDAQ | |
| Noncumulative Preferred Stock | ||
| Document Information [Line Items] | ||
| Title of 12(b) Security | Depositary Shares (each representing a 1/40th interest in a share of 6.75% Series A Fixed-Rate Non-Cumulative perpetual preferred stock) | |
| Trading Symbol | FGBIP | |
| Security Exchange Name | NASDAQ |