COMMITMENTS AND CONTINGENCIES - FERC ROE Complaints (Details) - USD ($) $ in Millions			9 Months Ended	36 Months Ended
	Apr. 29, 2016	Oct. 16, 2014	Sep. 30, 2021	Oct. 15, 2014	Dec. 31, 2020
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 First, Second and Third Complaints | Minimum
Loss Contingencies [Line Items]
Percentage of maximum ROE for any incentive project				11.14%
FERC ROE First, Second and Third Complaints | Maximum
Loss Contingencies [Line Items]
Percentage of maximum ROE for any incentive project				13.10%
FERC ROE Complaints
Loss Contingencies [Line Items]
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 Fourth Complaint
Loss Contingencies [Line Items]
Base ROE percentage of complaint period	10.57%
Maximum percentage cap of ROE for incentive projects	11.74%
FERC ROE First Complaint
Loss Contingencies [Line Items]
Base ROE subsequently authorized		10.57%
Percentage of maximum ROE for any incentive project		11.74%
Amounts associated with first complaint refunded			38.9
FERC ROE First Complaint | CL&P
Loss Contingencies [Line Items]
Amounts associated with first complaint refunded			22.4
FERC ROE First Complaint | NSTAR Electric
Loss Contingencies [Line Items]
Amounts associated with first complaint refunded			13.7
FERC ROE First Complaint | PSNH
Loss Contingencies [Line Items]
Amounts associated with first complaint refunded			$ 2.8
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		$ 39.1
FERC ROE Second Complaint | CL&P
Loss Contingencies [Line Items]
Loss contingency, estimate of possible earnings impact			21.4		21.4
FERC ROE Second Complaint | NSTAR Electric
Loss Contingencies [Line Items]
Loss contingency, estimate of possible earnings impact			14.6		14.6
FERC ROE Second Complaint | PSNH
Loss Contingencies [Line Items]
Loss contingency, estimate of possible earnings impact			$ 3.1		$ 3.1