COMMITMENTS AND CONTINGENCIES - Eversource and NSTAR Electric Boston Harbor Civil Action (Details) $ in Millions		12 Months Ended
	Jul. 15, 2016 kV	Dec. 31, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Site Contingency [Line Items]
Amount of capacity required for installation of distribution cable (in kV) | kV	115
Asset retirement obligations		$ 466.2	$ 499.7	$ 489.5
Long-Term Derivative Liabilities
Site Contingency [Line Items]
Asset retirement obligations				32.0
NSTAR Electric Company
Site Contingency [Line Items]
Base rate credit for new cable		17.5
Asset retirement obligations		$ 72.4	$ 91.8	$ 97.5