PENSION AND OTHER POSTRETIREMENT BENEFITS - ESTIMATED COSTS THAT WILL BE AMORTIZED (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost/(Credit) amortized from Regulatory Assets into net periodic benefit costs	$ 203
Net Actuarial Loss amortized from Regulatory Assets into net periodic benefit costs	5,488
Prior Service Cost/(Credit) amortized from Accumulated Other Comprehensive Loss into net periodic benefit costs	9
Net Actuarial Loss amortized from Accumulated Other Comprehensive Loss into net periodic benefit costs	3,747
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior Service Cost/(Credit) amortized from Regulatory Assets into net periodic benefit costs	(257)
Net Actuarial Loss amortized from Regulatory Assets into net periodic benefit costs	998
Prior Service Cost/(Credit) amortized from Accumulated Other Comprehensive Loss into net periodic benefit costs	(88)
Net Actuarial Loss amortized from Accumulated Other Comprehensive Loss into net periodic benefit costs	$ 179