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Cover Page - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Feb. 10, 2020
Jun. 28, 2019
Cover page.      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2019    
Document Transition Report false    
Entity File Number 1-13883    
Entity Registrant Name CALIFORNIA WATER SERVICE GROUP    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 77-0448994    
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    
Title of 12(b) Security Common Stock, $0.01 par value per share    
Trading Symbol CWT    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 2,437
Entity Common Stock, Shares Outstanding   48,536,524  
Documents Incorporated by Reference
Certain information required to be disclosed in Part III of this report is incorporated by reference from the registrant’s definitive Proxy Statement for its Annual Meeting of Stockholders to be held on or about May 27, 2020. The proxy statement is expected to be filed no later than 120 days after the end of the fiscal year covered by this report.
    
Entity Central Index Key 0001035201    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY