SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Net income	$ 100,979	$ 96,831	$ 63,116
Net loss attributable to noncontrolling interests	(146)	0	0
Net income attributable to California Water Service Group	$ 101,125	$ 96,831	$ 63,116
Weighted average common shares, basic (in shares)	51,633	49,274	48,168
Weighted average common shares, dilutive (in shares)	51,633	49,274	48,168
Earnings per share—basic (in dollars per share)	$ 1.96	$ 1.97	$ 1.31
Earnings per share—diluted (in dollars per share)	$ 1.96	$ 1.97	$ 1.31