ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), beginning balance	$ (5,493)	$ (4,715)
Pension and other postretirement benefit plans obligations [Abstract]
Other comprehensive income (loss), before reclassification, net of tax	596	(1,293)
Amounts reclassified from accumulated other comprehensive income (loss), net of tax	811	515
Other comprehensive income (loss), net of tax	1,407	(778)	$ (314)
Accumulated Other Comprehensive Income (Loss), ending balance	(4,086)	(5,493)	(4,715)
Pension and other postretirement benefit plans obligations [Abstract]
Amortization of net loss	43,800	42,591	40,912
Amortization of unrecognized prior service cost	(4,567)	(4,597)	(3,316)
Other comprehensive income (loss) reclassification adjustment, before tax and regulatory adjustments	39,233	37,994	37,596
Other comprehensive income (loss) reclassification - regulatory adjustments	(38,151)	(37,157)	(36,845)
Other comprehensive income (loss) reclassification adjustment, before tax	1,082	837	751
Other comprehensive income (loss) reclassification adjustment, Tax	(271)	(322)	(289)
Other comprehensive income (loss) reclassification adjustment, net of tax	$ 811	$ 515	$ 462