COMMITMENTS AND CONTINGENCIES - Eversource and NSTAR Electric Boston Harbor Civil Action (Details) $ in Millions		12 Months Ended
	Jul. 15, 2016 kV	Dec. 31, 2019 USD ($) kV	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Site Contingency [Line Items]
Amount of capacity required for installation of distribution cable (in kV) | kV	115	115
Asset retirement obligations		$ 489.5	$ 466.2	$ 419.1
NSTAR Electric
Site Contingency [Line Items]
Base rate credit for new cable			17.5
Asset retirement obligations		97.5	$ 72.4	$ 44.6
Long-Term Derivative Liabilities
Site Contingency [Line Items]
Asset retirement obligations		32.0
Long-Term Derivative Liabilities | NSTAR Electric
Site Contingency [Line Items]
Asset retirement obligations		$ 32.0