LONG-TERM DEBT (Details) - USD ($) $ in Thousands		12 Months Ended
	Oct. 04, 2011	Dec. 31, 2017	Dec. 31, 2016	Nov. 17, 2010	Apr. 17, 2009
Long-term debt
Unamortized debt issuance costs		$ (3,889)	$ (4,475)
Total long-term debt		531,713	557,953
Current maturities of long-term debt		15,920	26,208
Long-term debt, less current maturities		515,793	531,745
Cal Water
Long-term debt
Current maturities of long-term debt		15,598	25,657
Long-term debt, less current maturities		514,952	530,850
Debt maturity period	15 years
Capital lease liability		6,400	7,000
First Mortgage
Long-term debt
Long-term debt		$ 521,445	547,000
First Mortgage Bonds TTT Series Due 2056 [Member]
Long-term debt
Interest rate		4.61%
Long-term debt		$ 10,000	10,000
First Mortgage Bonds SSS Series Due 2046 [Member]
Long-term debt
Interest rate		4.41%
Long-term debt		$ 40,000	40,000
First Mortgage Bonds QQQ Series Due 2025
Long-term debt
Interest rate		3.33%
Long-term debt		$ 50,000	50,000
First Mortgage Bonds RRR Series Due 2045
Long-term debt
Interest rate		4.31%
Long-term debt		$ 50,000	50,000
First Mortgage Bonds PPP Series, due 2040
Long-term debt
Interest rate		5.50%
Long-term debt		$ 100,000	100,000
First Mortgage Bonds PPP Series, due 2040 | Cal Water
Long-term debt
Interest rate				5.50%
First Mortgage Bonds LL Series, due 2019
Long-term debt
Interest rate		5.875%
Long-term debt		$ 100,000	100,000
First Mortgage Bonds LL Series, due 2019 | Cal Water
Long-term debt
Interest rate					5.875%
First Mortgage Bonds AAA Series, due in 2025
Long-term debt
Interest rate		7.28%
Long-term debt		$ 20,000	20,000
First Mortgage Bonds BBB Series, due in 2028
Long-term debt
Interest rate		6.77%
Long-term debt		$ 20,000	20,000
First Mortgage Bonds CCC Series, Due 2030
Long-term debt
Interest rate		8.15%
Long-term debt		$ 20,000	20,000
First Mortgage Bonds DDD Series, Due 2031
Long-term debt
Interest rate		7.13%
Long-term debt		$ 20,000	20,000
First Mortgage Bonds EEE Series, Due 2032
Long-term debt
Interest rate		7.11%
Long-term debt		$ 20,000	20,000
First Mortgage Bonds FFF Series, Due 2017
Long-term debt
Interest rate		5.90%
Long-term debt		$ 0	20,000
First Mortgage Bonds FFF Series, Due 2017 | Cal Water
Long-term debt
Repayment of debt		$ 20,000
First Mortgage Bonds GGG Series, Due 2022
Long-term debt
Interest rate		5.29%
Long-term debt		$ 9,091	10,909
First Mortgage Bonds HHH Series, Due 2022
Long-term debt
Interest rate		5.29%
Long-term debt		$ 9,091	10,909
First Mortgage Bonds III Series, Due 2023
Long-term debt
Interest rate		5.54%
Long-term debt		$ 5,454	6,364
First Mortgage Bonds JJJ Series, Due 2018
Long-term debt
Interest rate		5.44%
Long-term debt		$ 909	1,818
First Mortgage Bonds LLL Series, Due 2018
Long-term debt
Interest rate		5.48%
Long-term debt		$ 10,000	10,000
First Mortgage Bonds OOO Series, Due 2031
Long-term debt
Interest rate		6.02%
Long-term debt		$ 20,000	20,000
First Mortgage Bonds CC Series, Due 2020
Long-term debt
Interest rate		9.86%
Long-term debt		$ 16,900	17,000
California Department of Water Resources Loans
Long-term debt
Long-term debt		6,201	6,519
Other Long-term Debt
Long-term debt
Long-term debt		$ 7,956	$ 8,909
Minimum | California Department of Water Resources Loans
Long-term debt
Interest rate		2.60%	2.60%
Maximum | California Department of Water Resources Loans
Long-term debt
Interest rate		8.00%	8.00%