SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Accounting Policies [Abstract]
Net income	$ 190,085	$ 51,376	$ 95,263
Net loss attributable to noncontrolling interest	(722)	(535)	(748)
Net income attributable to California Water Service Group	$ 190,807	$ 51,911	$ 96,011
Weighted average common shares, basic (in shares)	58,612	56,952	54,320
Weighted average common shares, diluted (in shares)	58,647	56,983	54,363
Earnings per share—basic (in dollars per share)	$ 3.26	$ 0.91	$ 1.77
Earnings per share—diluted (in dollars per share)	$ 3.25	$ 0.91	$ 1.77