PENSION AND OTHER POSTRETIREMENT BENEFITS - DETAILS OF ACTIVITY WITHIN THE REGULATORY ASSET AND ACCUMULATED OTHER COMPREHENSIVE LOSS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Regulatory Assets
Beginning Balance	$ 64,432	$ 71,177
Amounts Arising During the Period, Net Actuarial (Gains)/Losses	9,706	(463)
Amounts Arising During the Period, Prior Service (Cost) Credit	0	0
Amounts Amortized to Net Periodic Costs, Net Actuarial Loss	(5,485)	(6,079)
Amounts Amortized to Net Periodic Costs, Prior Service (Cost) Credit	(203)	(203)
Ending Balance	68,450	64,432
Accumulated Other Comprehensive Loss (pre-tax)
Beginning Balance	20,463	23,170
Amounts Arising During the Period, Net Actuarial (Gains)/Losses	6,129	184
Amounts Arising During the Period, Prior Service Cost (Credit)	0	0
Amounts Amortized to Net Periodic Costs, Net Actuarial Loss	(2,490)	(2,891)
Amounts Amortized to Net Periodic Costs, Prior Service (Cost) Credit	0	0
Ending Balance	24,102	20,463
Other Postretirement Benefits
Regulatory Assets
Beginning Balance	15,347	27,863
Amounts Arising During the Period, Net Actuarial (Gains)/Losses	2,584	(3,155)
Amounts Arising During the Period, Prior Service (Cost) Credit	257	(499)
Amounts Amortized to Net Periodic Costs, Net Actuarial Loss	(945)	(1,107)
Amounts Amortized to Net Periodic Costs, Prior Service (Cost) Credit	0	(7,755)
Ending Balance	17,243	15,347
Accumulated Other Comprehensive Loss (pre-tax)
Beginning Balance	0	0
Amounts Arising During the Period, Net Actuarial (Gains)/Losses	0	0
Amounts Arising During the Period, Prior Service Cost (Credit)	0	0
Amounts Amortized to Net Periodic Costs, Net Actuarial Loss	0	0
Amounts Amortized to Net Periodic Costs, Prior Service (Cost) Credit	0	0
Ending Balance	$ 0	$ 0