COMMITMENTS AND CONTINGENCIES - Narrative (Details)		12 Months Ended
	Dec. 20, 2017 USD ($) well	Dec. 31, 2020 USD ($) acrefoot entity	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Long-term purchase commitments - other disclosures
Operating lease renewal term		25 years
Finance lease renewal term		25 years
Rent expense				$ 2,000,000.0
Contingency loss recognized liability		$ 2,600,000
Minimum
Long-term purchase commitments - other disclosures
Operating lease term		1 year
Finance lease term		1 year
Maximum
Long-term purchase commitments - other disclosures
Operating lease term		50 years
Finance lease term		50 years
Santa Clara Valley Water District
Long-term purchase commitments - other disclosures
Annual cost		$ 15,200,000	$ 13,600,000	9,700,000
Stockton East Water District (SEWD)
Long-term purchase commitments - other disclosures
Annual cost		$ 14,300,000	$ 13,300,000	$ 13,700,000
Kern County Water Agency (Agency)
Long-term purchase commitments - other disclosures
Minimum acre feet of treated water to be purchased per year over life of contract | acrefoot		20,500
Minimum acre feet of treated water to be purchased under prior agreement | acrefoot		11,500
Number of other parties obligated to purchase treated water | entity		3
Minimum acre feet of treated water to be purchased per year by other parties | acrefoot		32,500
Total obligation of all parties, excluding the Company		$ 82,400,000
Total capital facilities charge and treated water charge obligation		9,800,000
Total capital facilities charge and treated water charge obligation per acre foot		476.83
Total treated water charge		3,700,000
Portion of estimated operating cost per acre foot for treated water delivered		55.8
California Water Service Company and City of Bakersfield v. The Dow Chemical Company, et al., Civil Case No. CIV-470999 (TCP Action)
Long-term purchase commitments - other disclosures
Amount awarded	$ 85,000,000.0
Proceeds from legal settlements	$ 56,000,000.0	$ 46,700,000
Claims settled | well	47
Total claims | well	57
Release from future claims | well	36