SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 6) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets and Liabilities
Regulatory Assets	$ 401,147	$ 355,930
Regulatory Liabilities	179,706	38,735
Short-term portion of the regulatory assets	36,783	30,332
Short-term portion of the regulatory liabilities	59,303	4,759
Long-Term Debt Premium, Discount and Expense
Amortization of debt premium and expenses	920	871	$ 825
Advances for Construction
Advances for construction	$ 182,502	182,448
Refund period	40 years
Estimated refunds of advances
2018	$ 7,900
2019	7,700
2020	7,700
2021	7,600
2022	7,600
Future tax benefits due ratepayers
Regulatory Assets and Liabilities
Regulatory Liabilities	168,343	33,231
Health care balancing account
Regulatory Assets and Liabilities
Regulatory Liabilities	7,749	0
Conservation program
Regulatory Assets and Liabilities
Regulatory Liabilities	2,273	584
Pension balancing account
Regulatory Assets and Liabilities
Regulatory Liabilities	364	695
Net WRAM and MCBA long-term payable
Regulatory Assets and Liabilities
Regulatory Liabilities	513	611
Other Liabilities
Regulatory Assets and Liabilities
Regulatory Liabilities	464	3,614
Pension and retiree group health
Regulatory Assets and Liabilities
Regulatory Assets	214,249	188,880
Future tax benefits due ratepayers
Regulatory Assets and Liabilities
Regulatory Assets	87,323	92,099
Other accrued benefits
Regulatory Assets and Liabilities
Regulatory Assets	28,251	27,503
Net WRAM and MCBA long-term accounts receivable
Regulatory Assets and Liabilities
Regulatory Assets	34,879	16,148
Asset retirement obligations, net
Regulatory Assets and Liabilities
Regulatory Assets	17,126	15,812
Interim rates long-term accounts receivable
Regulatory Assets and Liabilities
Regulatory Assets	4,568	4,605
Tank Coating
Regulatory Assets and Liabilities
Regulatory Assets	10,998	8,452
Health care balancing account
Regulatory Assets and Liabilities
Regulatory Assets	496	1,000
Pension balancing account
Regulatory Assets and Liabilities
Regulatory Assets	2,322	0
Other regulatory assets
Regulatory Assets and Liabilities
Regulatory Assets	$ 935	$ 1,431