SHORT-TERM AND LONG-TERM DEBT - Commercial Paper Programs and Credit Agreements (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Eversource Parent [Member]
Line of Credit Facility [Line Items]
Amount outstanding during period	$ 0	$ 0
Eversource Parent [Member] | Commercial Paper [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	1,450,000,000
Short-term borrowings outstanding	698,500,000	1,100,000,000
Remaining borrowing capacity	$ 751,500,000	$ 351,500,000
Weighted average interest rate (as a percentage)	0.66%	0.72%
Eversource Parent [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 1,450,000,000
Debt instrument term	5 years
The Connecticut Light And Power Company [Member]
Line of Credit Facility [Line Items]
Notes payable to Eversource Parent	$ 108,500,000	$ 277,400,000
Public Service Company Of New Hampshire [Member]
Line of Credit Facility [Line Items]
Notes payable to Eversource Parent	107,500,000	231,300,000
Western Massachusetts Electric Company [Member]
Line of Credit Facility [Line Items]
Notes payable to Eversource Parent	48,200,000	143,400,000
NSTAR Electric Company [Member]
Line of Credit Facility [Line Items]
Amount outstanding during period	0	0
NSTAR Electric Company [Member] | Commercial Paper [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	450,000,000
Short-term borrowings outstanding	36,000,000	62,500,000
Remaining borrowing capacity	$ 414,000,000	$ 387,500,000
Weighted average interest rate (as a percentage)	0.42%	0.40%
NSTAR Electric Company [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 450,000,000
Debt instrument term	5 years