SHORT-TERM BORROWINGS (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Jun. 29, 2011
Short-term borrowings
Maximum borrowing capacity			$ 400,000,000
Basis of interest rate	LIBOR
Borrowings under the bank lines of credit
Maximum short-term borrowings	89,475,000	47,140,000
Average amount outstanding	71,715,000	32,324,000
Weighted average interest rate (as a percent)	1.74%	2.41%
Interest rate at end of period (as a percent)	2.14%	2.10%
Minimum
Short-term borrowings
Interest rate margin (as a percent)	0.725%
Commitment fee (as a percent)	0.08%
Maximum
Short-term borrowings
Interest rate margin (as a percent)	0.95%
Commitment fee (as a percent)	0.125%
California Water Service Group
Short-term borrowings
Maximum borrowing capacity			100,000,000
Amount outstanding under line of credit	64,500,000	47,100,000
Cal Water
Short-term borrowings
Maximum borrowing capacity			300,000,000
Period within which borrowings are to be repaid (in months)	12
Amount outstanding under line of credit	$ 25,000,000