| Document and Entity Information | Aug. 17, 2021 |
| Document And Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Aug. 17, 2021 |
| Entity Registrant Name | Fiesta Restaurant Group, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-35373 |
| Entity Tax Identification Number | 90-0712224 |
| Entity Address, Address Line One | 14800 Landmark Boulevard, Suite 500 |
| Entity Address, City or Town | Dallas |
| Entity Address, State or Province | TX |
| Entity Address, Postal Zip Code | 75254 |
| City Area Code | 972 |
| Local Phone Number | 702-9300 |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | FRGI |
| Security Exchange Name | NASDAQ |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001534992 |
| Amendment Flag | false |
