COMMITMENTS AND CONTINGENCIES - FERC ROE Complaints (Details) $ in Millions					9 Months Ended	12 Months Ended		15 Months Ended	30 Months Ended	36 Months Ended
	Oct. 06, 2017	Jun. 08, 2017	Jul. 15, 2016 MW	Apr. 29, 2016	Sep. 30, 2018	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 26, 2014	Apr. 14, 2017	Oct. 15, 2014
Loss Contingencies [Line Items]
Loss Contingency, Percentage Of Base ROE Reasonably Justified					10.41%
Loss Contingency, Percentage Of Incentive Cap On Total ROE					13.08%
Capacity required for installation of distribution cable | MW			0.115
FERC ROE First, Second and Third Complaints
Loss Contingencies [Line Items]
Base ROE percentage challenged by complainants						11.14%
Loss Contingency, Base ROE Subsequently Authorized by FERC for First Complaint Period and Prospectively From October 2014					11.14%
Percentage of ROE for any incentive project		13.08%
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 First Complaint
Loss Contingencies [Line Items]
Percentage of ROE for any incentive project									11.74%
Refund payments						38.9
FERC ROE Second Complaint
Loss Contingencies [Line Items]
Percentage of base ROE of complaint period								10.57%	10.57%
Loss contingency, estimate of possible earnings impact						39.1
Percentage of ROE for any incentive project								11.74%
FERC ROE Fourth Complaint
Loss Contingencies [Line Items]
Percentage of base ROE of complaint period				10.57%
Maximum percentage cap of ROE for incentive projects				11.74%
Original base ROE authorized by FERC at time of complaint	11.74%
Percentage of ROE for any incentive project	10.57%
The Connecticut Light And Power Company | FERC ROE First Complaint
Loss Contingencies [Line Items]
Refund payments						22.4
The Connecticut Light And Power Company | FERC ROE Second Complaint
Loss Contingencies [Line Items]
Loss contingency, estimate of possible earnings impact						21.4
NSTAR Electric Company
Loss Contingencies [Line Items]
Loss in period							$ 4.9
Base rate credit							$ 17.5
NSTAR Electric Company | FERC ROE First Complaint
Loss Contingencies [Line Items]
Refund payments						13.7
NSTAR Electric Company | FERC ROE Second Complaint
Loss Contingencies [Line Items]
Loss contingency, estimate of possible earnings impact						14.6
Public Service Company Of New Hampshire | FERC ROE First Complaint
Loss Contingencies [Line Items]
Refund payments						2.8
Public Service Company Of New Hampshire | FERC ROE Second Complaint
Loss Contingencies [Line Items]
Loss contingency, estimate of possible earnings impact						$ 3.1
Minimum
Loss Contingencies [Line Items]
Loss Contingency, Percentage Of Base ROE Reasonably Justified					9.60%
Minimum | FERC ROE First, Second and Third Complaints
Loss Contingencies [Line Items]
Percentage of ROE for any incentive project										11.14%
Maximum
Loss Contingencies [Line Items]
Loss Contingency, Percentage Of Base ROE Reasonably Justified					10.99%
Maximum | FERC ROE First, Second and Third Complaints
Loss Contingencies [Line Items]
Percentage of ROE for any incentive project										13.10%