SHORT-TERM AND LONG-TERM DEBT - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Apr. 30, 2021	Mar. 22, 2021	Mar. 18, 2021	Feb. 10, 2021	Dec. 31, 2020	May 15, 2020
Line of Credit Facility [Line Items]
Current portion of long-term debt	$ 1,211,191,000					$ 1,053,186,000
NSTAR Electric
Line of Credit Facility [Line Items]
Current portion of long-term debt	250,000,000					250,000,000
Notes payable to Eversource parent	22,500,000					21,300,000
Amount of long-term debt requested for issuance	1,600,000,000
NSTAR Electric | Senior Notes | NSTAR Electric 3.50% Series F Senior Notes | Subsequent Event
Line of Credit Facility [Line Items]
Interest rate, stated percentage		3.50%
Face amount		$ 250,000,000
CL&P
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	32,100,000
Amount of long-term debt requested for issuance			$ 350,000,000
PSNH
Line of Credit Facility [Line Items]
Current portion of long-term debt	160,000,000					282,000,000
Notes payable to Eversource parent	196,800,000					46,300,000
Amount of long-term debt requested for issuance				$ 350,000,000
PSNH | First Mortgage | PSNH 3.20% Series R First Mortgage Bonds | Subsequent Event
Line of Credit Facility [Line Items]
Interest rate, stated percentage		3.20%
Face amount		$ 160,000,000
Aquarion
Line of Credit Facility [Line Items]
Current portion of long-term debt	40,000,000.0
Amount of long-term debt requested for issuance					$ 100,000,000
Commercial Paper
Line of Credit Facility [Line Items]
Short-term debt	1,475,700,000					1,054,300,000
Commercial Paper | NSTAR Electric
Line of Credit Facility [Line Items]
Maximum borrowing capacity	650,000,000
Short-term debt	443,500,000					195,000,000.0
Commercial Paper | Aquarion
Line of Credit Facility [Line Items]
Short-term debt	46,800,000
Line of Credit
Line of Credit Facility [Line Items]
Amount outstanding under line of credit	0					$ 0
Line of Credit | NSTAR Electric
Line of Credit Facility [Line Items]
Maximum borrowing capacity	650,000,000
Eversource Parent | Commercial Paper
Line of Credit Facility [Line Items]
Maximum borrowing capacity	2,000,000,000.00
Eversource Parent | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 1,450,000,000
Debt instrument term	5 years
NSTAR Electric | Line of Credit | NSTAR Electric
Line of Credit Facility [Line Items]
Debt instrument term	5 years
Line of Credit | CL&P
Line of Credit Facility [Line Items]
Maximum borrowing capacity							$ 450,000,000
Amount outstanding under line of credit	$ 0
Line of Credit | PSNH
Line of Credit Facility [Line Items]
Maximum borrowing capacity							$ 300,000,000
Amount outstanding under line of credit	0
Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 550,000,000