SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 7) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets and Liabilities
Regulatory Assets	$ 433,322	$ 353,569
Regulatory Liabilities	211,413	211,275
Short-term portion of the regulatory assets	38,225	42,394
Short-term portion of the regulatory liabilities	4,462	12,213
Long-Term Debt Premium, Discount and Expense
Amortization of debt premium and expenses	744	1,099	$ 920
Advances for Construction
Advances for construction	$ 191,062	186,342
Refund period	40 years
2020	$ 7,628
2021	7,627
2022	7,627
2023	7,627
2024	7,627
Thereafter	152,926
Total refunds	191,062
Future tax benefits due to customers
Regulatory Assets and Liabilities
Regulatory Liabilities	194,501	180,205
Health cost balancing account
Regulatory Assets and Liabilities
Regulatory Liabilities	4,271	3,516
Conservation program
Regulatory Assets and Liabilities
Regulatory Liabilities	2,742	6,880
Net WRAM and MCBA long-term payable
Regulatory Assets and Liabilities
Regulatory Liabilities	211	222
Tax accounting memorandum account
Regulatory Assets and Liabilities
Regulatory Liabilities	806	5,039
Cost of capital memorandum account
Regulatory Assets and Liabilities
Regulatory Liabilities	151	2,834
1,2,3 trichloropropane settlement proceeds
Regulatory Assets and Liabilities
Regulatory Liabilities	8,426	12,142
Other regulatory liabilities
Regulatory Assets and Liabilities
Regulatory Liabilities	305	437
Pension and retiree group health
Regulatory Assets and Liabilities
Regulatory Assets	208,321	156,947
Property-related temporary differences (tax benefits flowed through to customers)
Regulatory Assets and Liabilities
Regulatory Assets	104,931	99,376
Other accrued benefits
Regulatory Assets and Liabilities
Regulatory Assets	20,030	20,588
Net WRAM and MCBA long-term accounts receivable
Regulatory Assets and Liabilities
Regulatory Assets	25,465	17,134
Asset retirement obligations, net
Regulatory Assets and Liabilities
Regulatory Assets	$ 19,567	18,197
Interim rates long-term accounts receivable
Regulatory Assets and Liabilities
Recovery Period	1 year
Regulatory Assets	$ 4,642	4,642
Tank coating
Regulatory Assets and Liabilities
Recovery Period	10 years
Regulatory Assets	$ 13,535	11,196
Recoverable property losses
Regulatory Assets and Liabilities
Recovery Period	10 years
Regulatory Assets	$ 5,000	1,275
Pension cost balancing account
Regulatory Assets and Liabilities
Recovery Period	1 year
Regulatory Assets	$ 21,465	16,494
Other components of net periodic benefit cost
Regulatory Assets and Liabilities
Regulatory Assets	5,145	3,221
Other regulatory assets
Regulatory Assets and Liabilities
Regulatory Assets	$ 5,221	$ 4,499
Minimum | Net WRAM and MCBA long-term accounts receivable
Regulatory Assets and Liabilities
Recovery Period	1 year
Maximum | Net WRAM and MCBA long-term accounts receivable
Regulatory Assets and Liabilities
Recovery Period	2 years