SHORT-TERM AND LONG-TERM DEBT - Narrative (Details)		1 Months Ended	9 Months Ended	12 Months Ended
	Oct. 10, 2018 USD ($)	Jan. 31, 2018 USD ($) issuance	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Notes payable to Eversource parent			$ 196,900,000	$ 115,900,000
Long-term debt, current maturities			387,310,000	549,631,000
Commercial Paper
Line of Credit Facility [Line Items]
Maximum borrowing capacity				201,200,000.0
Line of Credit
Line of Credit Facility [Line Items]
Amount outstanding during period			0	0
Eversource Parent | Commercial Paper
Line of Credit Facility [Line Items]
Maximum borrowing capacity			1,450,000,000	201,200,000.0
Short-term debt			826,700,000	979,300,000
Long-term debt, current maturities		$ 446,800,000
Number of debt issuances | issuance		2
Eversource Parent | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity			$ 1,450,000,000
Debt instrument term			5 years
NSTAR Electric Company
Line of Credit Facility [Line Items]
Notes payable to Eversource parent			$ 16,000,000	198,000,000
NSTAR Electric Company | Commercial Paper
Line of Credit Facility [Line Items]
Maximum borrowing capacity			650,000,000
Short-term debt			240,500,000	234,000,000
NSTAR Electric Company | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity			$ 650,000,000
Debt instrument term			5 years
Aquarion
Line of Credit Facility [Line Items]
Notes payable to Eversource parent			$ 108,000,000
Aquarion | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity			100,000,000.0
Amount outstanding during period			0
Short-term debt				76,000,000
The Connecticut Light and Power Company
Line of Credit Facility [Line Items]
Notes payable to Eversource parent			45,900,000	69,500,000
Long-term debt, current maturities			250,000,000	300,000,000
Public Service Company of New Hampshire
Line of Credit Facility [Line Items]
Notes payable to Eversource parent			46,600,000	262,900,000
Long-term debt, current maturities			0	$ 110,000,000
NSTAR Gas
Line of Credit Facility [Line Items]
Notes payable to Eversource parent			$ 117,200,000
2001 Series A Pollution Control Revenue Bonds | Subsequent Event | Public Service Company of New Hampshire
Line of Credit Facility [Line Items]
Extinguishment of Debt, Amount	$ 89,300,000