SHORT-TERM AND LONG-TERM DEBT - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
CL&P
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	$ 261,600,000
PSNH
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	61,000,000	$ 57,000,000
HEEC
Line of Credit Facility [Line Items]
Notes payable to Eversource parent	22,300,000
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	$ 100,000,000.0
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
Commercial paper borrowings	261,600,000
Eversource Parent | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 1,450,000,000
Debt instrument term	5 years