SHORT-TERM AND LONG-TERM DEBT - Commercial Paper Programs and Credit Agreements (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
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	801,000,000	1,000,000,000
Remaining borrowing capacity	$ 649,000,000	$ 428,000,000
Weighted average interest rate (as a percentage)	1.12%	0.88%
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	$ 3,400,000	$ 80,100,000
Public Service Company Of New Hampshire [Member]
Line of Credit Facility [Line Items]
Notes payable to Eversource Parent	144,900,000	160,900,000
Western Massachusetts Electric Company [Member]
Line of Credit Facility [Line Items]
Notes payable to Eversource Parent	71,400,000	51,000,000
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	174,500,000	126,500,000
Remaining borrowing capacity	$ 275,500,000	$ 323,500,000
Weighted average interest rate (as a percentage)	0.86%	0.71%
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