PENSION AND OTHER POSTRETIREMENT BENEFITS - ESTIMATED COSTS THAT WILL BE AMORTIZED (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)
Pension Benefits
Regulatory Assets into Net Periodic Benefit Costs
Prior Service Cost/(Credit)	$ 127
Net Actuarial Loss	6,202
Accumulated Other Comprehensive Income (Loss) into Net Periodic Benefit Costs
Prior Service Cost/(Credit)	5
Net Actuarial Loss	3,943
Other Postretirement Benefits
Regulatory Assets into Net Periodic Benefit Costs
Prior Service Cost/(Credit)	(257)
Net Actuarial Loss	1,040
Accumulated Other Comprehensive Income (Loss) into Net Periodic Benefit Costs
Prior Service Cost/(Credit)	(87)
Net Actuarial Loss	$ 207