ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pension and other postretirement benefit plans, Tax	$ 486	$ (130)
Accumulated Other Comprehensive Income (Loss), beginning balance	(4,401)	(5,174)
Pension and other postretirement benefit plans obligations [Abstract]
Other comprehensive income (loss), before reclassification, net of tax	(776)	209
Amounts reclassified from accumulated other comprehensive income (loss), net of tax	462	564
Other comprehensive loss	(314)	773	$ (1,781)
Accumulated Other Comprehensive Income (Loss), ending balance	(4,715)	(4,401)	(5,174)
Pension and other postretirement benefit plans obligations [Abstract]
Amortization of net loss	40,912	47,494	34,169
Amortization of unrecognized prior service cost	(3,316)	(1,962)	(1,211)
Other comprehensive income (loss) reclassification adjustment, before tax and regulatory adjustments	37,596	45,532	32,958
Other comprehensive income (loss) reclassification - regulatory adjustments	(36,845)	(44,615)	(32,445)
Other comprehensive income (loss) reclassification adjustment, before tax	751	917	513
Other comprehensive income (loss) reclassification adjustment, Tax	(289)	(353)	(198)
Other comprehensive income (loss) reclassification adjustment, net of tax	$ 462	$ 564	$ 315