LONG-TERM DEBT (Details) (USD $)			1 Months Ended								1 Months Ended																																					12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Oct. 31, 2011 Cal Water Y	Dec. 31, 2012 Cal Water	Dec. 31, 2011 Cal Water	Oct. 04, 2011 Cal Water	Dec. 31, 2012 First Mortgage Bonds	Dec. 31, 2011 First Mortgage Bonds	Dec. 31, 2012 First Mortgage Bonds, 5.500% due 2040	Dec. 31, 2011 First Mortgage Bonds, 5.500% due 2040	Nov. 30, 2010 First Mortgage Bonds, 5.500% due 2040 Cal Water	Nov. 17, 2010 First Mortgage Bonds, 5.500% due 2040 Cal Water	Dec. 31, 2012 First Mortgage Bonds, 5.875% due 2019	Dec. 31, 2011 First Mortgage Bonds, 5.875% due 2019	Apr. 17, 2009 First Mortgage Bonds, 5.875% due 2019 Cal Water	Dec. 31, 2012 First Mortgage Bonds, AAA Series, due in 2025	Dec. 31, 2011 First Mortgage Bonds, AAA Series, due in 2025	Dec. 31, 2012 First Mortgage Bonds, BBB Series, due in 2028	Dec. 31, 2011 First Mortgage Bonds, BBB Series, due in 2028	Dec. 31, 2012 First Mortgage Bonds, CCC Series, due in 2030	Dec. 31, 2011 First Mortgage Bonds, CCC Series, due in 2030	Dec. 31, 2012 First Mortgage Bonds, DDD Series, due in 2031	Dec. 31, 2011 First Mortgage Bonds, DDD Series, due in 2031	Dec. 31, 2012 First Mortgage Bonds, EEE Series, due in 2032	Dec. 31, 2011 First Mortgage Bonds, EEE Series, due in 2032	Dec. 31, 2012 First Mortgage Bonds, FFF Series, due in 2017	Dec. 31, 2011 First Mortgage Bonds, FFF Series, due in 2017	Dec. 31, 2012 First Mortgage Bonds, GGG Series, due in 2022	Dec. 31, 2011 First Mortgage Bonds, GGG Series, due in 2022	Dec. 31, 2012 First Mortgage Bonds, HHH Series, due in 2022	Dec. 31, 2011 First Mortgage Bonds, HHH Series, due in 2022	Dec. 31, 2012 First Mortgage Bonds, III Series, due in 2023	Dec. 31, 2011 First Mortgage Bonds, III Series, due in 2023	Dec. 31, 2012 First Mortgage Bonds, JJJ Series, due in 2018	Dec. 31, 2011 First Mortgage Bonds, JJJ Series, due in 2018	Dec. 31, 2012 First Mortgage Bonds, LLL Series, due in 2018	Dec. 31, 2011 First Mortgage Bonds, LLL Series, due in 2018	Dec. 31, 2012 First Mortgage Bonds, MMM Series, due in 2013	Dec. 31, 2011 First Mortgage Bonds, MMM Series, due in 2013	Dec. 31, 2012 First Mortgage Bonds, NNN Series, due in 2013	Dec. 31, 2011 First Mortgage Bonds, NNN Series, due in 2013	Dec. 31, 2012 First Mortgage Bonds, OOO Series, due in 2031	Dec. 31, 2011 First Mortgage Bonds, OOO Series, due in 2031	Dec. 31, 2012 First Mortgage Bonds, CC Series, due in 2020	Dec. 31, 2011 First Mortgage Bonds, CC Series, due in 2020	Dec. 31, 2012 First Mortgage Bonds, K Series, due in 2012	Dec. 31, 2011 First Mortgage Bonds, K Series, due in 2012	Dec. 31, 2012 California Department of Water Resources Loans	Dec. 31, 2011 California Department of Water Resources Loans	Dec. 31, 2012 Other Long-term debt	Dec. 31, 2011 Other Long-term debt
Long-term debt
Interest Rate (as a percent)									5.50%	5.50%		5.50%	5.875%	5.875%	5.875%	7.28%	7.28%	6.77%	6.77%	8.15%	8.15%	7.13%	7.13%	7.11%	7.11%	5.90%	5.90%	5.29%	5.29%	5.29%	5.29%	5.54%	5.54%	5.44%	5.44%	5.48%	5.48%	5.52%	5.52%	5.55%	5.55%	6.02%	6.02%	9.86%	9.86%	6.94%	6.94%
Interest rate, low end of range (as a percent)																																																2.60%	2.60%
Interest rate, high end of range (as a percent)																																																8.00%	8.00%
Total long-term debt	$ 481,250,000	$ 488,165,000					$ 459,219,000	$ 464,664,000	$ 100,000,000	$ 100,000,000			$ 100,000,000	$ 100,000,000		$ 20,000,000	$ 20,000,000	$ 20,000,000	$ 20,000,000	$ 20,000,000	$ 20,000,000	$ 20,000,000	$ 20,000,000	$ 20,000,000	$ 20,000,000	$ 20,000,000	$ 20,000,000	$ 18,182,000	$ 20,000,000	$ 18,182,000	$ 20,000,000	$ 10,000,000	$ 10,000,000	$ 5,455,000	$ 6,364,000	$ 10,000,000	$ 10,000,000	$ 20,000,000	$ 20,000,000	$ 20,000,000	$ 20,000,000	$ 20,000,000	$ 20,000,000	$ 17,400,000	$ 17,500,000		$ 800,000	$ 8,451,000	$ 8,780,000	$ 13,580,000	$ 14,721,000
Less current maturities	46,783,000	6,533,000		46,104,000	5,851,000
Long-term debt excluding current maturities	434,467,000	481,632,000		431,433,000	477,998,000
Debt maturity period (in years)			15
Capital lease liability						9,200,000
Sale and issuance of First Mortgage Bonds											$ 100,000,000