COMMITMENTS AND CONTINGENCIES - Schedule of Complaints and Base ROEs (Details) - USD ($) $ in Millions					3 Months Ended	15 Months Ended			16 Months Ended	30 Months Ended	36 Months Ended
	Mar. 27, 2018	Oct. 06, 2017	Jun. 08, 2017	Apr. 29, 2016	Mar. 31, 2018	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 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 March 31, 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 March 31, 2018 (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%
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 March 31, 2018 (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)		11.74%				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 March 31, 2018 (in millions)					0.0
FERC ALJ Recommendation of base ROE on Second and Third Complaints (Issued March 22, 2016) (in percentage)						10.57%
Percentage of maximum ROE for any incentive project		10.57%
Base ROE percentage of complaint period	10.57%			10.57%
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 March 31, 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 March 31, 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 March 31, 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%