EMPLOYEE BENEFITS - Schedule of Assumptions used To Calculate Pension and SERP and PBOP (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Expected Long-Term Rate of Return (in percentage)	8.25%	8.25%	8.25%
Compensation/Progression Rate (in percentage)	3.50%	3.50%	3.50%
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Discount Rate (in percentage)			4.22%
Expected Long-Term Rate of Return (in percentage)	8.25%	8.25%	8.25%
Minimum | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Discount Rate (in percentage)	3.20%	3.27%	4.20%
Minimum | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Discount Rate (in percentage)	3.48%	2.88%
Maximum | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Discount Rate (in percentage)	3.90%	4.89%
Maximum | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Discount Rate (in percentage)	4.64%	4.09%