Cover	3 Months Ended
	Mar. 31, 2020 shares
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2020
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 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 Common Stock, Shares Outstanding	48,714,000
Entity Central Index Key	0001035201
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1