SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 6) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 Y employee	Dec. 31, 2011	Dec. 31, 2010
Regulatory Assets and Liabilities
Regulatory assets	$ 344,419	$ 319,898
Regulatory assets, net long-term receivable	344,419	319,898
Regulatory liability	35,720	28,037
Short-term portion of the regulatory assets	34,020	21,680
Short-term portion of the regulatory liabilities	5,018	2,655
Long-Term Debt Premium, Discount and Expense
Amortization expense included in interest expense	1,107	1,082	979
Advances for Construction
Advances for construction refunded primarily over a 40-year period	186,753	185,902
Refund period (in years)	40
Other advances for construction refundable based upon customer connections	831	1,376
Estimated refunds of advances
2013	7,179
2014	7,178
2015	7,115
2016	7,098
2017	7,081
Thereafter	151,102
Collective Bargaining Agreements
Number of employees	1,131
Number of non-supervisory employees represented by the Utility Workers Union of America	734
Future tax benefits due ratepayers
Regulatory Assets and Liabilities
Regulatory liability	24,932	16,978
Conservation program
Regulatory Assets and Liabilities
Regulatory liability	6,538	4,328
Pension balancing account
Regulatory Assets and Liabilities
Regulatory liability		1,936
Other liabilities
Regulatory Assets and Liabilities
Regulatory liability	4,250	4,795
Pension and retiree group health
Regulatory Assets and Liabilities
Regulatory assets	223,153	213,819
Income tax temporary differences
Regulatory Assets and Liabilities
Regulatory assets	56,991	34,664
Other accrued benefits
Regulatory Assets and Liabilities
Regulatory assets	40,362	31,453
Net WRAM and MCBA accounts receivable
Regulatory Assets and Liabilities
Regulatory assets	42,071	49,625
Regulatory assets, net long-term receivable	12,051	30,268
Short-term portion of the regulatory assets	34,020	19,357
Asset retirement obligations, net
Regulatory Assets and Liabilities
Regulatory assets	11,862	9,694
Impairment of Long-Lived Assets, Intangibles and Goodwill
Construction costs removed from rate base		$ 320	$ 634