SHORT-TERM AND LONG-TERM DEBT - Commercial Paper Programs and Credit Agreements (Details)	1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 31, 2018 USD ($) issuance	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Notes payable to Eversource parent		$ 276,300,000	$ 115,900,000
Long-term debt, current maturities		1,097,288,000	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		$ 682,300,000	$ 979,300,000
Weighted-average interest rate		2.31%	1.86%
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			$ 198,000,000
NSTAR Electric Company | Commercial Paper
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 650,000,000
Short-term debt		$ 367,000,000	$ 234,000,000
Weighted-average interest rate		1.75%	1.55%
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		0	69,500,000
Long-term debt, current maturities		550,000,000	300,000,000
Public Service Company Of New Hampshire
Line of Credit Facility [Line Items]
Notes payable to Eversource parent		271,300,000	262,900,000
Long-term debt, current maturities		$ 110,000,000	$ 110,000,000