SHORT-TERM AND LONG-TERM DEBT - Narrative (Details) - USD ($)	6 Months Ended
	Jun. 30, 2021	Jul. 31, 2021	May 18, 2021	Mar. 31, 2021	Dec. 31, 2020
NSTAR Electric
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	$ 21,500,000				$ 21,300,000
Amount of long-term debt requested for issuance				$ 1,600,000,000
CL&P | Bonds | Subsequent Event
Line of Credit Facility [Line Items]
Interest rate, stated percentage		4.375%
Face amount		$ 120,500,000
PSNH
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	48,600,000				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
Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	550,000,000
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	2,000,000,000.00
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