SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accounting Policies [Abstract]
Net income (loss)	$ 51,376	$ 95,263	$ 100,979
Net loss attributable to noncontrolling interests	(535)	(748)	(146)
Net income attributable to California Water Service Group	$ 51,911	$ 96,011	$ 101,125
Weighted average common shares, basic (in shares)	56,952	54,320	51,633
Diluted (in shares)	56,983	54,363	51,633
Earnings per share—basic (in dollars per share)	$ 0.91	$ 1.77	$ 1.96
Earnings per share—diluted (in dollars per share)	$ 0.91	$ 1.77	$ 1.96