COMMITMENTS AND CONTINGENCIES - FERC ROE Complaints (Details) - USD ($) $ in Millions			9 Months Ended	15 Months Ended			36 Months Ended
	Apr. 29, 2016	Oct. 16, 2014	Sep. 30, 2018	Jul. 28, 2017	Mar. 26, 2014	Dec. 31, 2012	Oct. 15, 2014
Loss Contingencies [Line Items]
Percentage of base ROE reasonably justified			10.41%
Percentage of incentive cap on total ROE			13.08%
FERC ROE First, Second and Third Complaints
Loss Contingencies [Line Items]
Base ROE subsequently authorized			11.14%
FERC ROE Complaints
Loss Contingencies [Line Items]
Period of complaint			15 months
Basis point change			10.00%
Estimate of possible loss for each 10 basis point change to base ROE			$ 3.0
FERC ROE Fourth Complaint
Loss Contingencies [Line Items]
Base ROE percentage of complaint period	10.57%
Maximum percentage cap of ROE for incentive projects	11.74%
Loss contingency, estimate of possible earnings impact			0.0
Base ROE subsequently authorized				10.57%
FERC ROE First Complaint
Loss Contingencies [Line Items]
Percentage of maximum ROE for any incentive project		11.74%
Loss contingency, estimate of possible earnings impact			0.0
Base ROE subsequently authorized		10.57%				10.57%
FERC ROE Second Complaint
Loss Contingencies [Line Items]
Base ROE percentage of complaint period					10.57%
Percentage of maximum ROE for any incentive project					11.74%
Loss contingency, estimate of possible earnings impact			$ 39.1
Minimum
Loss Contingencies [Line Items]
Percentage of base ROE reasonably justified			9.60%
Minimum | FERC ROE First, Second and Third Complaints
Loss Contingencies [Line Items]
Percentage of maximum ROE for any incentive project							11.14%
Maximum
Loss Contingencies [Line Items]
Percentage of base ROE reasonably justified			10.99%
Maximum | FERC ROE First, Second and Third Complaints
Loss Contingencies [Line Items]
Percentage of maximum ROE for any incentive project							13.10%