SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 6) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 Y employee	Dec. 31, 2010	Dec. 31, 2009
Regulatory Assets and Liabilities
Regulatory Assets	$ 319,898	$ 229,577
Regulatory assets, net long-term receivable	319,898	229,577
Regulatory liability	28,037	17,079
Short-term portion of the regulatory assets	21,680	14,784
Short-term portion of the regulatory liabilities	2,655	3,025
Long-Term Debt Premium, Discount and Expense
Amortization expense included in interest expense	1,082	979	970
Advances for Construction
Advances for construction refunded primarily over a 40-year period	185,902	185,332
Refund period (in years)	40
Other advances for construction refundable based upon customer connections	1,376	1,567
Estimated refunds of advances
2012	7,011
2013	6,914
2014	6,908
2015	6,850
2016	6,847
Thereafter	152,748
Collective Bargaining Agreements
Number of employees	1,132
Number of non-supervisory employees represented by the Utility Workers Union of America	703
Future tax benefits due ratepayers
Regulatory Assets and Liabilities
Regulatory liability	16,978	15,253
Conservation program liability
Regulatory Assets and Liabilities
Regulatory liability	4,328
Pension balancing account
Regulatory Assets and Liabilities
Regulatory liability	1,936
Other liabilities
Regulatory Assets and Liabilities
Regulatory liability	4,795	1,826
Pension and retiree group health
Regulatory Assets and Liabilities
Regulatory Assets	213,819	145,451
Income tax temporary differences
Regulatory Assets and Liabilities
Regulatory Assets	34,664	30,934
Other accrued benefits
Regulatory Assets and Liabilities
Regulatory Assets	31,453	29,919
Net WRAM and MCBA accounts receivable
Regulatory Assets and Liabilities
Regulatory Assets	49,625	31,570
Regulatory assets, net long-term receivable	30,268	16,786
Short-term portion of the regulatory assets	21,680	14,784
Asset retirement obligations, net
Regulatory Assets and Liabilities
Regulatory Assets	9,694	6,487
Impairment of Long-Lived Assets, Intangibles and Goodwill
Construction costs removed from rate base	$ 320	$ 634