COMMITMENTS AND CONTINGENCIES - FERC ROE Complaints (Details) $ in Millions			12 Months Ended	30 Months Ended	36 Months Ended
	Oct. 16, 2018 complaint	Apr. 29, 2016 complaint	Dec. 31, 2021 USD ($)	Apr. 14, 2017	Oct. 15, 2014	Dec. 31, 2020 USD ($)	Jul. 31, 2014 complaint
Loss Contingencies [Line Items]
Percentage of base ROE reasonably justified	10.41%
Percentage of incentive cap on total ROE	13.08%
FERC ROE Complaints
Loss Contingencies [Line Items]
Loss contingency, pending claims, number | complaint	4	4
Period of complaint			15 months
Basis point change			0.10%
Estimate of possible loss for each 10 basis point change to base ROE			$ 3.0
FERC ROE First, Second and Third Complaints
Loss Contingencies [Line Items]
Loss contingency, pending claims, number | complaint							3
Base ROE percentage challenged by complainants			11.14%
Percentage of ROE for any incentive project	13.08%
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%
Percentage of ROE for any incentive project			11.74%
Loss contingency, estimate of possible earnings impact			$ 39.1
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%
Percentage of ROE for any incentive project			10.57%
Original base ROE authorized by FERC at time of complaint			11.74%
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			$ 21.4
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			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			$ 3.1
Minimum | FERC ROE First, Second and Third Complaints
Loss Contingencies [Line Items]
Percentage of ROE for any incentive project					11.14%
Maximum | FERC ROE First, Second and Third Complaints
Loss Contingencies [Line Items]
Percentage of ROE for any incentive project					13.10%