SHORT-TERM DEBT - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Dec. 28, 2016	Nov. 03, 2016	Aug. 08, 2016	Dec. 31, 2015	Jun. 16, 2015
Short-term Debt [Line Items]
Amount of debt authorized			$ 125,000,000
Notes payable	$ 1,148,500,000	$ 1,148,500,000				$ 1,160,953,000
The Connecticut Light And Power Company [Member]
Short-term Debt [Line Items]
Short-term borrowing limit approved by regulatory agency							$ 600,000,000
Notes payable to related parties	80,100,000	80,100,000				277,400,000
Western Massachusetts Electric Company [Member]
Short-term Debt [Line Items]
Short-term borrowing limit approved by regulatory agency							$ 300,000,000
Notes payable to related parties	51,000,000	51,000,000				143,400,000
NSTAR Electric Company [Member]
Short-term Debt [Line Items]
Short-term borrowing limit approved by regulatory agency					$ 655,000,000
Notes payable	126,500,000	126,500,000				62,500,000
NSTAR Electric Company [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Amount of commercial paper program	450,000,000	450,000,000
Amount available for borrowing under credit facility	$ 323,500,000	$ 323,500,000				$ 387,500,000
Weighted average interest rate (as a percentage)	0.71%	0.71%				0.40%
Short-term borrowings outstanding under commercial paper program	$ 126,500,000	$ 126,500,000				$ 62,500,000
NSTAR Electric Company [Member] | Line of Credit [Member]
Short-term Debt [Line Items]
Amount of commercial paper program	$ 450,000,000	$ 450,000,000
Debt instrument term		5 years
Period of extension of termination date	1 year
Amount outstanding during period		$ 0
NPT [Member]
Short-term Debt [Line Items]
Amount of debt authorized				$ 800,000,000
Public Service Company Of New Hampshire [Member]
Short-term Debt [Line Items]
Short-term borrowing limit approved by regulatory agency	$ 349,000,000	349,000,000
Notes payable to related parties	$ 160,900,000	$ 160,900,000				231,300,000
Public Service Company Of New Hampshire [Member] | Short Term Debt Authorization Calculation [Member]
Short-term Debt [Line Items]
Interest rate stated percentage (up to) (as a percentage)	10.00%	10.00%
Short-term debt borrowing calculation approved by regulatory agency	$ 60,000,000	$ 60,000,000
Eversource [Member]
Short-term Debt [Line Items]
Notes payable	1,022,000,000	1,022,000,000				1,098,453,000
Eversource [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Amount of commercial paper program	1,450,000,000	1,450,000,000
Notes payable	1,000,000,000	1,000,000,000				1,100,000,000
Amount available for borrowing under credit facility	$ 428,000,000	$ 428,000,000				$ 351,500,000
Weighted average interest rate (as a percentage)	0.88%	0.88%				0.72%
Eversource [Member] | Line of Credit [Member]
Short-term Debt [Line Items]
Amount of commercial paper program	$ 1,450,000,000	$ 1,450,000,000
Debt instrument term		5 years
Period of extension of termination date	1 year
Amount outstanding during period		$ 0
Unsecured Debt [Member] | The Connecticut Light And Power Company [Member]
Short-term Debt [Line Items]
Period of maturity restricting provisions of debt		10 years
Total capitalization (as a percentage)		10.00%
Capacity available under preferred stock provisions	$ 557,600,000	$ 557,600,000