SHORT-TERM AND LONG-TERM DEBT - Borrowings Outstanding and Available under the Commercial Paper Programs (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Debt Instrument [Line Items]
Borrowings Outstanding as of	$ 533,000	$ 1,525,445
Commercial Paper | Eversource Parent Commercial Paper Program
Debt Instrument [Line Items]
Borrowings Outstanding as of	533,000	1,280,000
Available Borrowing Capacity as of	$ 1,467,000	$ 720,000
Weighted-average interest rate (in percent)	4.08%	3.98%
Commercial Paper | NSTAR Electric
Debt Instrument [Line Items]
Borrowings Outstanding as of	$ 0	$ 245,400
Available Borrowing Capacity as of	$ 650,000	$ 404,600
Weighted-average interest rate (in percent)	0.00%	3.87%