ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pension and other postretirement benefit plans, Tax		$ (130)	$ 1,442
Accumulated Other Comprehensive Income (Loss), beginning balance	$ 0	(5,174)	0
Transfers in upon separation	$ 3,393
Pension and other postretirement benefit plans obligations [Abstract]
Other comprehensive income (loss), before reclassification, net of tax of $(130)		209	(2,096)
Amounts reclassified from accumulated other comprehensive income (loss), net of tax of $(353)		564	315
Other comprehensive loss		773	(1,781)	$ 0
Accumulated Other Comprehensive Income (Loss), ending balance		(4,401)	(5,174)	$ 0
Pension and other postretirement benefit plans obligations [Abstract]
Amortization of net loss		47,494	34,169
Amortization of unrecognized prior service cost		(1,962)	(1,211)
Other comprehensive income (loss) reclassification adjustment, before tax and regulatory adjustments		45,532	32,958
Other comprehensive income (loss) reclassification - regulatory adjustments		(44,615)	(32,445)
Other comprehensive income (loss) reclassification adjustment, before tax		917	513
Other comprehensive income (loss) reclassification adjustment, Tax		(353)	(198)
Other comprehensive income (loss) reclassification adjustment, net of tax		$ 564	$ 315