SHORT-TERM DEBT - Schedule of Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Notes Payable	$ 910,000	$ 1,088,087
Parent Company
Line of Credit Facility [Line Items]
Notes Payable	631,500	778,087
Commercial Paper | Parent Company
Line of Credit Facility [Line Items]
Notes Payable	631,500	979,300
Available Borrowing Capacity	$ 818,500	$ 470,700
Weighted-Average discount (as a percentage)	2.77%	1.86%
Commercial Paper | NSTAR Electric Company
Line of Credit Facility [Line Items]
Notes Payable	$ 278,500	$ 234,000
Available Borrowing Capacity	$ 371,500	$ 416,000
Weighted-Average discount (as a percentage)	2.50%	1.55%