COMMITMENTS AND CONTINGENCIES - Schedule of Complaints and Base ROEs (Details) - USD ($) $ in Millions			9 Months Ended	15 Months Ended			16 Months Ended	36 Months Ended
	Apr. 29, 2016	Oct. 16, 2014	Sep. 30, 2018	Jul. 28, 2017	Mar. 26, 2014	Dec. 31, 2012	Oct. 30, 2015	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 also Effective from October 16, 2014 through April 14, 2017 (as a percentage)		10.57%				10.57%
Reserve (Pre-Tax and Excluding Interest) as of September 30, 2018 (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, 2018 (in millions)			39.1
Percentage of maximum ROE for any incentive project					11.74%
Base ROE percentage of complaint period					10.57%
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 also Effective from October 16, 2014 through April 14, 2017 (as a percentage)							10.57%
Reserve (Pre-Tax and Excluding Interest) as of September 30, 2018 (in millions)			0.0
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 also Effective from October 16, 2014 through April 14, 2017 (as a percentage)				10.57%
Reserve (Pre-Tax and Excluding Interest) as of September 30, 2018 (in millions)			$ 0.0
Base ROE percentage of complaint period	10.57%
FERC ROE First, Second and Third Complaints
Loss Contingencies [Line Items]
Base ROE Subsequently Authorized by FERC for First Complaint Period and also Effective from October 16, 2014 through April 14, 2017 (as a percentage)			11.14%
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, 2018 (in millions)			21.4
NSTAR Electric Company | FERC ROE First Complaint
Loss Contingencies [Line Items]
Customer refund payments			13.7
NSTAR Electric Company | FERC ROE Second Complaint
Loss Contingencies [Line Items]
Reserve (Pre-Tax and Excluding Interest) as of September 30, 2018 (in millions)			14.6
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, 2018 (in millions)			$ 3.1
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%