COMMITMENTS AND CONTINGENCIES (Detail 2) $ in Thousands		12 Months Ended
	Dec. 20, 2017 USD ($) well	Dec. 31, 2017 USD ($) entity $ / acrefoot acrefoot	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Long-term purchase commitments - other disclosures
Proceeds from legal settlements		$ 56,004	$ 0	$ 0
Contingency loss recognized liability		6,100
Santa Clara Valley Water District
Long-term purchase commitments - other disclosures
Annual cost		9,100	8,500	6,300
Stockton East Water District (SEWD)
Long-term purchase commitments - other disclosures
Annual cost		$ 14,100	$ 12,200	$ 9,800
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
Total capital facilities charge and treated water charge obligation		$ 9,100
Total capital facilities charge and treated water charge obligation per acre foot | $ / acrefoot		441.9
Total treated water charge		$ 3,000
Portion of estimated operating cost per acre foot for treated water delivered | $ / acrefoot		64.2
California Water Service Company and City of Bakersfield v. The Dow Chemical Company, et al., Civil Case No. CIV-470999 (TCP Action) [Member]
Long-term purchase commitments - other disclosures
Amount awarded	$ 85,000
Proceeds from legal settlements	$ 56,000
Claims settled | well	47
Total claims | well	57
Release from future claims | well	36