SHORT-TERM BORROWINGS (Details) (USD $)	12 Months Ended			12 Months Ended				12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Oct. 27, 2009	Dec. 31, 2011 Minimum	Dec. 31, 2011 Maximum	Dec. 31, 2011 California Water Service Group	Dec. 31, 2010 California Water Service Group	Dec. 31, 2011 Cal Water M	Oct. 27, 2009 Cal Water	Jun. 29, 2011 Cal Water Maximum
Short-term borrowings
Unsecured revolving credit facilities						$ 47,100,000	$ 23,800,000			$ 400,000,000
Maximum borrowing capacity under the short-term credit facility			100,000,000						300,000,000
Basis of interest rate	LIBOR
Interest rate margin (as a percent)				0.725%	0.95%
Commitment fee (as a percent)				0.08%	0.125%
Period within which borrowings are to be repaid (in months)								12
Amount outstanding under line of credit						47,100,000	23,800,000			400,000,000
Borrowings under the bank lines of credit
Maximum short-term borrowings	47,140,000	80,250,000
Average amount outstanding	$ 32,324,000	$ 43,224,000
Weighted average interest rate (as a percent)	2.41%	3.01%
Interest rate at end of period (as a percent)	2.10%	2.74%