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. 31, 2017	Jul. 30, 2017	Mar. 26, 2014	Dec. 31, 2012	Oct. 30, 2015	Mar. 31, 2017	Oct. 15, 2014
FERC ROE First Complaint [Member]
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 March 31, 2017 (in millions)	$ 0.0					$ 0.0
Percentage of maximum ROE for any incentive project						11.74%
Customer Refund Payments	38.9
FERC ROE Second Complaint [Member]
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, 2017 (in millions)	39.1					$ 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 [Member]
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 March 31, 2017 (in millions)	0.0					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 [Member]
Loss Contingencies [Line Items]
Reserve (Pre-Tax and Excluding Interest) as of March 31, 2017 (in millions)	0.0					0.0
Scenario, Forecast [Member] | FERC ROE Fourth Complaint [Member]
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%
The Connecticut Light And Power Company [Member] | FERC ROE First Complaint [Member]
Loss Contingencies [Line Items]
Customer Refund Payments	22.4
The Connecticut Light And Power Company [Member] | FERC ROE Second Complaint [Member]
Loss Contingencies [Line Items]
Reserve (Pre-Tax and Excluding Interest) as of March 31, 2017 (in millions)	21.4					21.4
NSTAR Electric Company [Member] | FERC ROE First Complaint [Member]
Loss Contingencies [Line Items]
Customer Refund Payments	8.4
NSTAR Electric Company [Member] | FERC ROE Second Complaint [Member]
Loss Contingencies [Line Items]
Reserve (Pre-Tax and Excluding Interest) as of March 31, 2017 (in millions)	8.5					8.5
Public Service Company Of New Hampshire [Member] | FERC ROE First Complaint [Member]
Loss Contingencies [Line Items]
Customer Refund Payments	2.8
Public Service Company Of New Hampshire [Member] | FERC ROE Second Complaint [Member]
Loss Contingencies [Line Items]
Reserve (Pre-Tax and Excluding Interest) as of March 31, 2017 (in millions)	3.1					3.1
Western Massachusetts Electric Company [Member] | FERC ROE First Complaint [Member]
Loss Contingencies [Line Items]
Customer Refund Payments	5.3
Western Massachusetts Electric Company [Member] | FERC ROE Second Complaint [Member]
Loss Contingencies [Line Items]
Reserve (Pre-Tax and Excluding Interest) as of March 31, 2017 (in millions)	$ 6.1					$ 6.1
Minimum [Member] | FERC ROE First, Second and Third Complaints [Member]
Loss Contingencies [Line Items]
Percentage of maximum ROE for any incentive project							11.14%
Maximum [Member] | FERC ROE First, Second and Third Complaints [Member]
Loss Contingencies [Line Items]
Percentage of maximum ROE for any incentive project							13.10%