INCOME TAXES (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income taxes
Unrecognized tax benefits	$ 10,499	$ 10,298	$ 7,916	$ 612
Income tax deductions	72,000	60,000	45,200
Valuation allowance	0
Deferred tax liability bonus depreciation	347,071	309,088
Unrecognized tax benefits that would impact effective tax rate	2,300
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
Statutory income tax	26,422	24,607	29,649
Increase (reduction) in taxes due to:
State income taxes net of federal tax benefit	4,341	4,043	4,871
Effect of regulatory treatment of fixed asset differences	(4,298)	(3,450)	(5,541)
Investment tax credits	(74)	(74)	(74)
Other	425	163	(933)
Total income tax	26,816	$ 25,289	$ 27,972
State
Income taxes
Net operating loss resulting from repairs and maintenance deductions	49,200
Federal
Income taxes
Net operating loss resulting from repairs and maintenance deductions	52,100
California | State
Income taxes
Tax credits	3,700
Estimated carried-forward portion of tax credits	$ 2,200