COMMITMENTS AND CONTINGENCIES - Schedule of Complaints and Base ROEs (Details) - USD ($) $ in Millions			9 Months Ended	15 Months Ended			16 Months Ended	30 Months Ended	36 Months Ended
	Oct. 06, 2017	Jun. 08, 2017	Sep. 30, 2017	Jul. 28, 2017	Mar. 26, 2014	Dec. 31, 2012	Oct. 30, 2015	Apr. 14, 2017	Oct. 15, 2014
FERC ROE First Complaint
Loss Contingencies [Line Items]
Original Base ROE Authorized by FERC at Time of Complaint (in percentage)						11.14%
Base ROE Subsequently Authorized by FERC for first complaint period and prospectively from October 16, 2014 (in percentage)						10.57%
Reserve (Pre-Tax and Excluding Interest) as of September 30, 2017 (in millions)			$ 0.0
Percentage of maximum ROE for any incentive project								11.74%
Customer refund payments			38.9
FERC ROE Second Complaint
Loss Contingencies [Line Items]
Original Base ROE Authorized by FERC at Time of Complaint (in percentage)					11.14%
Reserve (Pre-Tax and Excluding Interest) as of September 30, 2017 (in millions)			39.1
FERC ALJ Recommendation of base ROE on Second and Third Complaints (Issued March 22, 2016) (in percentage)					9.59%
Percentage of maximum 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 (in percentage)							11.14%
Base ROE Subsequently Authorized by FERC for first complaint period and prospectively from October 16, 2014 (in percentage)							10.57%
Reserve (Pre-Tax and Excluding Interest) as of September 30, 2017 (in millions)			0.0
FERC ALJ Recommendation of base ROE on Second and Third Complaints (Issued March 22, 2016) (in percentage)							10.90%
FERC ROE Fourth Complaint
Loss Contingencies [Line Items]
Original Base ROE Authorized by FERC at Time of Complaint (in percentage)				10.57%
Base ROE Subsequently Authorized by FERC for first complaint period and prospectively from October 16, 2014 (in percentage)				10.57%
Reserve (Pre-Tax and Excluding Interest) as of September 30, 2017 (in millions)			0.0
FERC ROE First, Second and Third Complaints
Loss Contingencies [Line Items]
Percentage of maximum ROE for any incentive project		13.50%
The Connecticut Light And Power Company | FERC ROE First Complaint
Loss Contingencies [Line Items]
Customer refund payments			22.4
The Connecticut Light And Power Company | FERC ROE Second Complaint
Loss Contingencies [Line Items]
Reserve (Pre-Tax and Excluding Interest) as of September 30, 2017 (in millions)			21.4
NSTAR Electric Company | FERC ROE First Complaint
Loss Contingencies [Line Items]
Customer refund payments			8.4
NSTAR Electric Company | FERC ROE Second Complaint
Loss Contingencies [Line Items]
Reserve (Pre-Tax and Excluding Interest) as of September 30, 2017 (in millions)			8.5
Public Service Company Of New Hampshire | FERC ROE First Complaint
Loss Contingencies [Line Items]
Customer refund payments			2.8
Public Service Company Of New Hampshire | FERC ROE Second Complaint
Loss Contingencies [Line Items]
Reserve (Pre-Tax and Excluding Interest) as of September 30, 2017 (in millions)			3.1
Western Massachusetts Electric Company | FERC ROE First Complaint
Loss Contingencies [Line Items]
Customer refund payments			5.3
Western Massachusetts Electric Company | FERC ROE Second Complaint
Loss Contingencies [Line Items]
Reserve (Pre-Tax and Excluding Interest) as of September 30, 2017 (in millions)			$ 6.1
Minimum | FERC ROE First, Second and Third Complaints
Loss Contingencies [Line Items]
Percentage of maximum ROE for any incentive project									11.14%
Maximum | FERC ROE First, Second and Third Complaints
Loss Contingencies [Line Items]
Percentage of maximum ROE for any incentive project									13.10%
Subsequent Event | FERC ROE Fourth Complaint
Loss Contingencies [Line Items]
Original Base ROE Authorized by FERC at Time of Complaint (in percentage)	11.74%
Percentage of maximum ROE for any incentive project	10.57%