SHORT-TERM AND LONG-TERM DEBT - Narrative (Details) - USD ($)	9 Months Ended
	Sep. 30, 2021	Sep. 10, 2021	Mar. 31, 2021	Dec. 31, 2020
NSTAR Electric
Line of Credit Facility [Line Items]
Long-term Line of Credit, Noncurrent	$ 80,000,000
Notes payable to Eversource parent	24,600,000			$ 21,300,000
Amount of long-term debt requested for issuance			$ 1,600,000,000
PSNH
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	66,500,000			46,300,000
Massachusetts Department of Public Utilities
Line of Credit Facility [Line Items]
Amount of long-term debt requested for issuance		$ 725,000,000
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
Commercial Paper | NSTAR Electric
Line of Credit Facility [Line Items]
Maximum borrowing capacity	650,000,000
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
Eversource Parent | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 2,000,000,000
Debt instrument term	5 years
NSTAR Electric | Line of Credit | NSTAR Electric
Line of Credit Facility [Line Items]
Debt instrument term	5 years