LONG-TERM DEBT (Details) - USD ($)				12 Months Ended
	Jun. 11, 2019	Sep. 13, 2018	Oct. 04, 2011	Dec. 31, 2019	Dec. 31, 2018	Nov. 17, 2010
Long-term debt
Unamortized debt issuance costs				$ (4,693,000)	$ (3,762,000)
Total long-term debt				808,622,000	814,938,000
Current maturities of long-term debt, net				21,868,000	104,911,000
Long-term debt, net				786,754,000	710,027,000
Cal Water
Long-term debt
Debt maturity period			15 years
Capital lease liability				5,200,000	5,800,000
Mortgages [Member] | Cal Water
Long-term debt
Debt issued	$ 400,000,000.0
Mortgages [Member] | Series UUU | Cal Water
Long-term debt
Repayment of debt	$ 300,000,000.0
Mortgages [Member] | Series VVV | Cal Water
Long-term debt
Interest rate	3.40%
Debt issued	$ 100,000,000.0
Mortgages [Member] | Series WWW | Cal Water
Long-term debt
Interest rate	4.07%
Debt issued	$ 100,000,000.0
Mortgages [Member] | Series YYY | Cal Water
Long-term debt
Interest rate	4.17%
Debt issued	$ 200,000,000.0
First Mortgage
Long-term debt
Long-term debt				$ 801,246,000	805,892,000
First Mortgage Bonds YYY Series Due 2059
Long-term debt
Interest rate				4.17%
Long-term debt				$ 200,000,000	0
First Mortgage Bonds WWW Series Due 2049
Long-term debt
Interest rate				4.07%
Long-term debt				$ 100,000,000	0
First Mortgage Bonds VVV Series Due 2029
Long-term debt
Interest rate				3.40%
Long-term debt				$ 100,000,000	0
First Mortgage Bonds TTT Series Due 2056
Long-term debt
Interest rate				4.61%
Long-term debt				$ 10,000,000	10,000,000
First Mortgage Bonds SSS Series Due 2046
Long-term debt
Interest rate				4.41%
Long-term debt				$ 40,000,000	40,000,000
First Mortgage Bonds QQQ Series Due 2025
Long-term debt
Interest rate				3.33%
Long-term debt				$ 50,000,000	50,000,000
First Mortgage Bonds RRR Series Due 2045
Long-term debt
Interest rate				4.31%
Long-term debt				$ 50,000,000	50,000,000
First Mortgage Bonds PPP Series, due 2040
Long-term debt
Interest rate				5.50%
Long-term debt				$ 100,000,000	100,000,000
First Mortgage Bonds PPP Series, due 2040 | Cal Water
Long-term debt
Interest rate						5.50%
Debt issued						$ 100,000,000.0
First Mortgage Bonds LL Series, due 2019
Long-term debt
Interest rate				5.875%
Long-term debt				$ 0	100,000,000
First Mortgage Bonds LL Series, due 2019 | Cal Water
Long-term debt
Repayments of Debt				100,000,000.0
First Mortgage Bonds UUU Series, due 2020
Long-term debt
Long-term debt				$ 0	300,000,000
First Mortgage Bonds AAA Series, due in 2025
Long-term debt
Interest rate				7.28%
Long-term debt				$ 20,000,000	20,000,000
First Mortgage Bonds BBB Series, due in 2028
Long-term debt
Interest rate				6.77%
Long-term debt				$ 20,000,000	20,000,000
First Mortgage Bonds CCC Series, Due 2030
Long-term debt
Interest rate				8.15%
Long-term debt				$ 20,000,000	20,000,000
First Mortgage Bonds DDD Series, Due 2031
Long-term debt
Interest rate				7.13%
Long-term debt				$ 20,000,000	20,000,000
First Mortgage Bonds EEE Series, Due 2032
Long-term debt
Interest rate				7.11%
Long-term debt				$ 20,000,000	20,000,000
First Mortgage Bonds GGG Series, Due 2022
Long-term debt
Interest rate				5.29%
Long-term debt				$ 5,455,000	7,273,000
First Mortgage Bonds HHH Series, Due 2022
Long-term debt
Interest rate				5.29%
Long-term debt				$ 5,455,000	7,273,000
First Mortgage Bonds III Series, Due 2023
Long-term debt
Interest rate				5.54%
Long-term debt				$ 3,636,000	4,546,000
First Mortgage Bonds OOO Series, Due 2031
Long-term debt
Interest rate				6.02%
Long-term debt				$ 20,000,000	20,000,000
First Mortgage Bonds CC Series, Due 2020
Long-term debt
Interest rate				9.86%
Long-term debt				$ 16,700,000	16,800,000
Secured Debt | First Mortgage Bonds due 2020
Long-term debt
Debt issued		$ 300,000,000.0
Secured Debt | LIBOR | First Mortgage Bonds due 2020
Long-term debt
Interest rate margin (as a percent)		0.70%
California Department of Water Resources Loans
Long-term debt
Long-term debt				$ 5,604,000	5,830,000
California Department of Water Resources Loans | Minimum
Long-term debt
Interest rate				3.00%
California Department of Water Resources Loans | Maximum
Long-term debt
Interest rate				7.00%
Other Long-term Debt
Long-term debt
Long-term debt				$ 6,465,000	$ 6,978,000
First Mortgage Bond JJJ and LLL | Cal Water
Long-term debt
Repayment of debt				$ 10,900,000