ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of net loss	$ 10,950	$ 10,648	$ 32,850	$ 31,944
Amortization of unrecognized prior service cost	(1,142)	(1,149)	(3,426)	(3,447)
Reclassification adjustment, before tax and regulatory adjustments	9,808	9,499	29,424	28,497
Regulatory adjustments	(9,537)	(9,290)	(28,611)	(27,869)
Reclassification adjustment, before tax	271	209	813	628
Reclassification adjustment, Tax	(68)	(81)	(487)	(242)
Reclassification adjustment, net of tax	$ 203	$ 128	$ 326	$ 386