INCOME TAXES (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income taxes
Unrecognized tax benefits	$ 7,916,000	$ 612,000		$ 831,000
Income tax deductions	45,200,000	94,800,000	102,200,000
Deferred income tax Liability	15,800,000	33,200,000
Valuation allowance	0
Additional first year bonus depreciation (as a percent)	50.00%
Deferred tax liability bonus depreciation	272,958,000	231,141,000
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	1,600,000
Reconciliation of the income tax expense computed by applying the current federal tax rate to pretax book income and the amount shown in the Consolidated Statements of Income
Computed expected tax expense	29,649,000	23,708,000	24,600,000
Increase (reduction) in taxes due to:
State income taxes net of federal tax benefit	4,871,000	3,895,000	4,041,000
Effect of regulatory treatment of fixed asset differences	(5,541,000)	(4,112,000)	(7,030,000)
State tax credits		(2,465,000)
Investment tax credits	(74,000)	(74,000)	(74,000)
Other	(933,000)	(483,000)	(85,000)
Total	27,972,000	20,469,000	21,452,000
Benefit from tax accounting method change	6,274,000	3,329,000	7,029,000
State
Income taxes
Net operating loss resulting from repairs and maintenance deductions	49,900,000
Federal
Income taxes
Income tax deductions	20,400,000	16,100,000	14,600,000
Net operating loss resulting from repairs and maintenance deductions	43,800,000
Deferred tax liability bonus depreciation	7,100,000	5,600,000
California | State
Income taxes
Tax credits		4,000,000
Tax credit carry-forward period	10 years
Estimated carried-forward portion of tax credits	$ 2,300,000