SHORT-TERM DEBT - Schedule of Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Notes Payable	$ 1,088,087	$ 1,148,500
Revolving Credit Facility (1)
Line of Credit Facility [Line Items]
Notes Payable	76,000
Available Borrowing Capacity	$ 24,000
Weighted-Average discount (as a percentage)	2.66%
Parent Company
Line of Credit Facility [Line Items]
Notes Payable	$ 778,087	1,022,000
Commercial Paper | Parent Company
Line of Credit Facility [Line Items]
Notes Payable	979,300	1,022,000
Available Borrowing Capacity	$ 470,700	$ 428,000
Weighted-Average discount (as a percentage)	1.86%	0.88%
NSTAR Electric Company
Line of Credit Facility [Line Items]
Notes Payable	$ 234,000	$ 126,500
NSTAR Electric Company | Commercial Paper
Line of Credit Facility [Line Items]
Notes Payable	234,000	126,500
Available Borrowing Capacity	$ 416,000	$ 323,500
Weighted-Average discount (as a percentage)	1.55%	0.71%