SHORT-TERM AND LONG-TERM DEBT - Narrative (Details) - USD ($)	9 Months Ended
	Sep. 30, 2019	Aug. 14, 2019	Apr. 26, 2019	Dec. 31, 2018
CL&P
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	$ 173,100,000
Amount of long-term debt approved for issuance		$ 675,000,000
PSNH
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	42,100,000			$ 57,000,000.0
Amount of long-term debt approved for issuance			$ 300,000,000
HEEC
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	48,700,000
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 | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 1,450,000,000
Debt instrument term	5 years