COMMITMENTS AND CONTINGENCIES - Eversource and NSTAR Electric Boston Harbor Civil Action (Details) $ in Millions		12 Months Ended
	Jul. 15, 2016 USD ($) kV	Dec. 31, 2018 USD ($)	Sep. 30, 2019 USD ($)
Loss Contingencies [Line Items]
Amount of capacity required for installation of distribution cable (in kV) | kV	115
NSTAR Electric
Loss Contingencies [Line Items]
Base rate credit for new cable	$ 17.5	$ 17.5
Other Long-Term Liabilities
Loss Contingencies [Line Items]
Asset Retirement Obligation			$ 32.0
Other Long-Term Liabilities | NSTAR Electric
Loss Contingencies [Line Items]
Asset Retirement Obligation			$ 32.0