SHORT-TERM DEBT - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Jan. 08, 2018	Dec. 08, 2017	Nov. 30, 2017	Nov. 03, 2016
Commercial Paper
Short-term Debt [Line Items]
Amount of commercial paper program	$ 201,200,000.0	$ 0
Eversource | Commercial Paper
Short-term Debt [Line Items]
Amount of commercial paper program	1,450,000,000			$ 1,450,000,000.00
Eversource | Line of Credit
Short-term Debt [Line Items]
Amount of commercial paper program	$ 1,450,000,000.00
Debt instrument term	5 years
Amount outstanding during period	$ 0	0
The Connecticut Light And Power Company
Short-term Debt [Line Items]
Short-term borrowing limit approved by regulatory agency					$ 600,000,000
Notes payable to related parties	69,500,000	80,100,000
NSTAR Electric Company
Short-term Debt [Line Items]
Short-term borrowing limit approved by regulatory agency					$ 655,000,000
Notes payable to related parties	0	51,000,000
NSTAR Electric Company | Commercial Paper
Short-term Debt [Line Items]
Amount of commercial paper program	650,000,000			450,000,000
NSTAR Electric Company | Line of Credit
Short-term Debt [Line Items]
Amount of commercial paper program	$ 650,000,000			$ 450,000,000
Debt instrument term	5 years
NPT
Short-term Debt [Line Items]
Amount of debt authorized						$ 800,000,000
Public Service Company Of New Hampshire
Short-term Debt [Line Items]
Short-term borrowing limit approved by regulatory agency	$ 364,000,000
Notes payable to related parties	$ 262,900,000	$ 160,900,000
Public Service Company Of New Hampshire | Short Term Debt Authorization Calculation
Short-term Debt [Line Items]
Interest rate stated percentage (up to) (as a percentage)	10.00%
Short-term debt borrowing calculation approved by regulatory agency	$ 60,000,000
Acquarion | Line of Credit
Short-term Debt [Line Items]
Amount of commercial paper program	$ 100,000,000
Unsecured Debt | The Connecticut Light And Power Company
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	$ 607,400,000
Subsequent Event | Eversource | Commercial Paper
Short-term Debt [Line Items]
Amount of commercial paper program			$ 201,200,000.0