INCOME TAXES (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income taxes
Federal income tax expense	$ 28,178,000	$ 20,395,000	$ 19,757,000
Deferred tax liability bonus depreciation	207,642,000	155,916,000
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	24,600,000	21,308,000	21,774,000
Increase (reduction) in taxes due to:
State income taxes net of federal tax benefit	4,041,000	3,500,000	3,577,000
Effect of regulatory treatment of fixed asset differences	(7,030,000)	(1,614,000)
Investment tax credits	(74,000)	(74,000)	(74,000)
Other	(85,000)	46,000	(721,000)
Total	21,452,000	23,166,000	24,556,000
Federal
Income taxes
Federal income tax repairs deduction for qualified tangible property	100,700,000
Deferred Tax Liabilities	35,300,000
Increase (decrease) to income tax expense	831,000	(397,000)	(433,000)
Deferred tax liability bonus depreciation	1,610,000	12,600,000
Current federal tax rate (as a percent)	35.00%
Federal | Lesser of
Income taxes
Qualified US production activities deduction rate of taxable income (as a percent)	9.00%
Qualified US production activities deduction rate of taxable gross wages (as a percent)	50.00%
State
Income taxes
Federal income tax repairs deduction for qualified tangible property	136,200,000
Increase (decrease) to income tax expense	(7,800,000)
State | State of California
Income taxes
Increase (decrease) to income tax expense		$ (1,600,000)