ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), beginning balance	$ (4,086)	$ (5,493)
Pension and other postretirement benefit plans obligations [Abstract]
Other comprehensive income (loss), before reclassification, net of tax	(2,074)	596
Amounts reclassified from accumulated other comprehensive income (loss), net of tax	639	811
Other comprehensive income (loss), net of tax	(1,435)	1,407	$ (778)
Accumulated Other Comprehensive Income (Loss), ending balance	(6,739)	(4,086)	(5,493)
Pension and other postretirement benefit plans obligations [Abstract]
Amortization of net loss	35,283	43,800	42,591
Amortization of unrecognized prior service cost	(673)	(4,567)	(4,597)
Other comprehensive income (loss) reclassification adjustment, before tax and regulatory adjustments	34,610	39,233	37,994
Other comprehensive income (loss) reclassification - regulatory adjustments	(33,758)	(38,151)	(37,157)
Other comprehensive income (loss) reclassification adjustment, before tax	852	1,082	837
Other comprehensive income (loss) reclassification adjustment, Tax	(213)	(271)	(322)
Other comprehensive income (loss) reclassification adjustment, net of tax	639	$ 811	$ 515
Stranded Deferred Tax Remeasurement Benefit	$ (1,218)