SHORT-TERM DEBT - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 18, 2019	Oct. 25, 2019	Dec. 08, 2017
Commercial Paper
Short-term Debt [Line Items]
Amount of commercial paper program	$ 346,300,000	$ 0
Line of Credit
Short-term Debt [Line Items]
Amount outstanding during period	0	0
Eversource | Commercial Paper
Short-term Debt [Line Items]
Amount of commercial paper program	1,450,000,000				$ 1,450,000,000
Eversource | Line of Credit
Short-term Debt [Line Items]
Amount of commercial paper program	$ 1,450,000,000
Debt instrument term	5 years
CL&P
Short-term Debt [Line Items]
Short-term borrowing limit approved by regulatory agency				$ 600,000,000
Notes payable to related parties	$ 63,800,000	0
NSTAR Electric
Short-term Debt [Line Items]
Short-term borrowing limit approved by regulatory agency			$ 655,000,000
Notes payable to related parties	30,300,000	0
NSTAR Electric | Commercial Paper
Short-term Debt [Line Items]
Amount of commercial paper program	650,000,000
NSTAR Electric | Line of Credit
Short-term Debt [Line Items]
Amount of commercial paper program	$ 650,000,000
Debt instrument term	5 years
PSNH
Short-term Debt [Line Items]
Short-term borrowing limit approved by regulatory agency	$ 354,000,000
Notes payable to related parties	$ 27,000,000	$ 57,000,000
PSNH | 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
Unsecured Debt | CL&P
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	$ 738,100,000