| Document and Entity Information Document | Jan. 15, 2021 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Jan. 15, 2021 |
| Entity Registrant Name | SJW Group |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-8966 |
| Entity Tax Identification Number | 77-0066628 |
| Entity Address, Address Line One | 110 West Taylor Street, |
| Entity Address, City or Town | San Jose, |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 95110 |
| City Area Code | (408) |
| Local Phone Number | 279-7800 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.001 per share |
| Trading Symbol | SJW |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
| Amendment Flag | false |
| Entity Central Index Key | 0000766829 |
