SHORT-TERM AND LONG-TERM DEBT - Narrative (Details) - USD ($)	1 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 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	$ 272,000,000.0	$ 272,000,000.0				$ 63,800,000
PSNH
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	119,300,000	119,300,000				27,000,000.0
PSNH | Subsequent Event
Line of Credit Facility [Line Items]
Amount of long-term debt approved for issuance			$ 200,000,000
NSTAR Electric
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	38,100,000	38,100,000				30,300,000
NSTAR Gas
Line of Credit Facility [Line Items]
Amount of long-term debt approved for issuance					$ 270,000,000
Aquarion
Line of Credit Facility [Line Items]
Long-term debt redeemed	32,200,000
Commercial Paper | NSTAR Electric
Line of Credit Facility [Line Items]
Maximum borrowing capacity	650,000,000	650,000,000
Line of Credit
Line of Credit Facility [Line Items]
Amount outstanding under line of credit	0	0				0
Line of Credit | NSTAR Electric
Line of Credit Facility [Line Items]
Maximum borrowing capacity	650,000,000	$ 650,000,000
Debt instrument term		5 years
Amount outstanding under line of credit	0	$ 0				$ 0
Eversource Parent | Commercial Paper
Line of Credit Facility [Line Items]
Maximum borrowing capacity	1,450,000,000	1,450,000,000
Eversource Parent | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	1,450,000,000	$ 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	$ 0
Line of Credit | PSNH
Line of Credit Facility [Line Items]
Maximum borrowing capacity				$ 300,000,000