SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Accounting Policies [Abstract]
Net income	$ 127,848	$ 190,085	$ 51,376
Net loss attributable to noncontrolling interest	(363)	(722)	(535)
Net income attributable to California Water Service Group	$ 128,211	$ 190,807	$ 51,911
Weighted average common shares, basic (in shares)	59,570	58,612	56,952
Weighted average common shares, diluted (in shares)	59,633	58,647	56,983
Earnings per share—basic (in dollars per share)	$ 2.15	$ 3.26	$ 0.91
Earnings per share—diluted (in dollars per share)	$ 2.15	$ 3.25	$ 0.91