COMMITMENTS AND CONTINGENCIES - FERC ROE Complaints (Details) $ in Millions						12 Months Ended	15 Months Ended			16 Months Ended	30 Months Ended	36 Months Ended
	Oct. 06, 2017	Jun. 08, 2017	Apr. 14, 2017	Jul. 15, 2016 MW	Apr. 29, 2016	Dec. 31, 2017 USD ($)	Jul. 28, 2017	Mar. 26, 2014	Dec. 31, 2012	Oct. 30, 2015	Apr. 14, 2017	Oct. 15, 2014
Loss Contingencies [Line Items]
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%				11.14%
Percentage of ROE for any incentive project		13.50%
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 First Complaint
Loss Contingencies [Line Items]
Base ROE percentage challenged by complainants			11.14%
Original base ROE authorized by FERC at time of complaint									11.14%
Loss contingency, estimate of possible earnings impact						0.0
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%
Original base ROE authorized by FERC at time of complaint								11.14%
Loss contingency, estimate of possible earnings impact						39.1
FERC ALJ recommended base ROE percentage								9.59%
Percentage of ROE for any incentive project								11.74%
FERC ROE Third Complaint
Loss Contingencies [Line Items]
Original base ROE authorized by FERC at time of complaint										11.14%
Loss Contingency, Base ROE Subsequently Authorized by FERC for First Complaint Period and Prospectively From October 2014										10.57%
Loss contingency, estimate of possible earnings impact						0.0
FERC ALJ recommended base ROE percentage										10.90%
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%						10.57%
Loss Contingency, Base ROE Subsequently Authorized by FERC for First Complaint Period and Prospectively From October 2014							10.57%
Loss contingency, estimate of possible earnings impact						0.0
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
Amount expensed						11.1
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 | 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%