SHORT-TERM DEBT - Schedule of Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Line of Credit Facility [Line Items]
Borrowings Outstanding	$ 1,505,450	$ 1,249,325
Eversource Parent Commercial Paper Program
Line of Credit Facility [Line Items]
Borrowings Outstanding	1,342,950	1,054,325
Commercial Paper | Eversource Parent Commercial Paper Program
Line of Credit Facility [Line Items]
Borrowings Outstanding	1,343,000	1,054,300
Available Borrowing Capacity	$ 657,000	$ 945,700
Weighted-Average Interest Rate (as a percentage)	0.31%	0.25%
Commercial Paper | NSTAR Electric Company
Line of Credit Facility [Line Items]
Borrowings Outstanding	$ 162,500	$ 195,000
Available Borrowing Capacity	$ 487,500	$ 455,000
Weighted-Average Interest Rate (as a percentage)	0.14%	0.16%