SHORT-TERM DEBT - Schedule of Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Borrowings Outstanding	$ 889,084	$ 910,000
Eversource Parent Commercial Paper Program
Line of Credit Facility [Line Items]
Borrowings Outstanding	878,584	631,500
Commercial Paper | Eversource Parent Commercial Paper Program
Line of Credit Facility [Line Items]
Borrowings Outstanding	1,224,900	631,500
Available Borrowing Capacity	$ 225,100	$ 818,500
Weighted-Average Interest Rate (as a percentage)	1.98%	2.77%
Commercial Paper | NSTAR Electric
Line of Credit Facility [Line Items]
Borrowings Outstanding	$ 10,500	$ 278,500
Available Borrowing Capacity	$ 639,500	$ 371,500
Weighted-Average Interest Rate (as a percentage)	1.63%	2.50%