SHORT-TERM AND LONG-TERM DEBT - Narrative (Details) - USD ($)	6 Months Ended
	Jun. 30, 2019	Apr. 26, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Current portion of long-term debt	$ (779,289,000)		$ (837,319,000)
Long-term debt	13,039,180,000		12,248,743,000
NSTAR Electric
Line of Credit Facility [Line Items]
Current portion of long-term debt	(95,000,000)		0
Long-term debt	3,246,535,000		2,944,846,000
CL&P
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	259,400,000
Current portion of long-term debt	0		(250,000,000)
Long-term debt	3,309,455,000		3,004,016,000
PSNH
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	20,100,000		57,000,000.0
Amount of long-term debt approved for issuance		$ 300,000,000
Current portion of long-term debt	(150,000,000)		(150,000,000)
Long-term debt	951,834,000		655,173,000
HEEC
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	40,300,000
NSTAR Gas | Senior Notes | NSTAR Gas 3.74% Series Q First Mortgage Bonds | Adjustment
Line of Credit Facility [Line Items]
Current portion of long-term debt	(75,000,000)
Long-term debt	75,000,000
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
Line of Credit | Aquarion
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 100,000,000
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