SHORT-TERM DEBT - Schedule of Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Line of Credit Facility [Line Items]
Borrowings Outstanding	$ 1,249,325	$ 889,084
Eversource Parent Commercial Paper Program
Line of Credit Facility [Line Items]
Borrowings Outstanding	1,054,325	878,584
Commercial Paper | Eversource Parent Commercial Paper Program
Line of Credit Facility [Line Items]
Borrowings Outstanding	1,054,300	1,224,900
Available Borrowing Capacity	$ 945,700	$ 225,100
Weighted-Average Interest Rate (as a percentage)	0.25%	1.98%
Commercial Paper | NSTAR Electric Company
Line of Credit Facility [Line Items]
Borrowings Outstanding	$ 195,000	$ 10,500
Available Borrowing Capacity	$ 455,000	$ 639,500
Weighted-Average Interest Rate (as a percentage)	0.16%	1.63%