SHORT-TERM AND LONG-TERM DEBT - Narrative (Details) - USD ($)	9 Months Ended
	Sep. 30, 2025	Oct. 31, 2025	May 27, 2025	Mar. 26, 2025	Dec. 31, 2024
NSTAR Electric | Subsequent Event
Line of Credit Facility [Line Items]
Long-term debt		$ 305,400,000
CL&P
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	$ 0				$ 280,000,000
CL&P | Related Party
Line of Credit Facility [Line Items]
Notes payable to Eversource parent					280,000,000.0
CL&P | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	375,000,000
Amount outstanding under line of credit	0				0
PSNH
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	8,100,000				131,100,000
PSNH | Related Party
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	8,100,000				131,100,000
PSNH | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	250,000,000		$ 300,000,000
Amount outstanding under line of credit	0				0
Yankee Gas | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity				$ 360,000,000
Eversource Parent | Subsequent Event
Line of Credit Facility [Line Items]
Long-term debt		$ 595,000,000.0
Commercial Paper | NSTAR Electric
Line of Credit Facility [Line Items]
Maximum borrowing capacity	650,000,000.0
Line of Credit | NSTAR Electric
Line of Credit Facility [Line Items]
Maximum borrowing capacity	650,000,000.0
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	$ 2,000,000,000
Debt instrument term (in years)	5 years
NSTAR Electric | Line of Credit | NSTAR Electric
Line of Credit Facility [Line Items]
Debt instrument term (in years)	5 years
NSTAR Electric and Eversource | Line of Credit
Line of Credit Facility [Line Items]
Amount outstanding under line of credit	$ 0				$ 0