SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Accounting Policies [Abstract]
Net income (loss)	$ 95,263	$ 100,979	$ 96,831
Net loss attributable to noncontrolling interests	(748)	(146)	0
Net income attributable to California Water Service Group	$ 96,011	$ 101,125	$ 96,831
Weighted average common shares, basic (in shares)	54,320	51,633	49,274
Weighted average common shares, dilutive (in shares)	54,363	51,633	49,274
Earnings per share—basic (in dollars per share)	$ 1.77	$ 1.96	$ 1.97
Earnings per share—diluted (in dollars per share)	$ 1.77	$ 1.96	$ 1.97