| Cover | Feb. 25, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Feb. 25, 2021 |
| Entity File Number | 1-13883 |
| Entity Registrant Name | CALIFORNIA WATER SERVICE GROUP |
| Entity Central Index Key | 0001035201 |
| Entity Tax Identification Number | 77-0448994 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 1720 North First Street |
| Entity Address, City or Town | San Jose |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 95112 |
| City Area Code | 408 |
| Local Phone Number | 367-8200 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Shares, par value $0.01 |
| Trading Symbol | CWT |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
