SHORT-TERM AND LONG-TERM DEBT - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Jan. 27, 2020	Dec. 31, 2019
Line of Credit Facility [Line Items]
Issuance of Long-Term Debt	$ 750,000,000	$ 0
CL&P
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	219,000,000.0			$ 63,800,000
PSNH
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	84,700,000			27,000,000.0
NSTAR Electric
Line of Credit Facility [Line Items]
Issuance of Long-Term Debt	400,000,000	$ 0
Notes payable to Eversource parent	38,400,000			30,300,000
NSTAR Gas Company [Member]
Line of Credit Facility [Line Items]
Current portion of long-term debt	25,000,000.0
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
Issuance of Long-Term Debt	165,000,000.0
Eversource Parent | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 1,450,000,000
Debt instrument term	5 years