SHORT-TERM AND LONG-TERM DEBT - Narrative (Details)	1 Months Ended	6 Months Ended	12 Months Ended
	Jan. 31, 2018 USD ($) issuance	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Notes payable to Eversource parent		$ 221,600,000	$ 115,900,000
Long-term debt, current maturities		387,296,000	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		747,000,000	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		$ 4,000,000	198,000,000
NSTAR Electric Company | Commercial Paper
Line of Credit Facility [Line Items]
Maximum borrowing capacity		650,000,000
Short-term debt		443,800,000	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
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		24,000,000	69,500,000
Long-term debt, current maturities		250,000,000	300,000,000
NSTAR Gas
Line of Credit Facility [Line Items]
Notes payable to Eversource parent		149,000,000
Public Service Company of New Hampshire
Line of Credit Facility [Line Items]
Notes payable to Eversource parent		118,700,000	262,900,000
Long-term debt, current maturities		$ 0	$ 110,000,000