| Cover | Oct. 24, 2019 |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 24, 2019 |
| Entity File Number | 0-21423 |
| Entity Registrant Name | BJs RESTAURANTS INC |
| Entity Central Index Key | 0001013488 |
| Entity Tax Identification Number | 33-0485615 |
| Entity Incorporation, State or Country Code | CA |
| Entity Address, Address Line One | 7755 Center Avenue, Suite 300 |
| Entity Address, City or Town | Huntington Beach |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 92647 |
| City Area Code | 714 |
| Local Phone Number | 500-2400 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, No Par Value |
| Trading Symbol | BJRI |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
