SHORT-TERM AND LONG-TERM DEBT - Narrative (Details) - USD ($)	9 Months Ended
	Sep. 30, 2020	Nov. 01, 2020	Oct. 21, 2020	Jul. 31, 2020	May 15, 2020	Jan. 27, 2020	Dec. 31, 2019
CL&P
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	$ 235,900,000						$ 63,800,000
PSNH
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	29,100,000						27,000,000.0
Amount of long-term debt approved for issuance				$ 200,000,000
NSTAR Electric
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	36,600,000						30,300,000
NSTAR Gas
Line of Credit Facility [Line Items]
Amount of long-term debt approved for issuance						$ 270,000,000
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
Debt instrument term	5 years
Amount outstanding under line of credit	$ 0						$ 0
Eversource Parent | Commercial Paper
Line of Credit Facility [Line Items]
Maximum borrowing capacity	1,450,000,000
Eversource Parent | Commercial Paper | Subsequent Event
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
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
Revolving Credit Facility | Subsequent Event
Line of Credit Facility [Line Items]
Maximum borrowing capacity			$ 550,000,000