COMMITMENTS AND CONTINGENCIES COMMITMENTS AND CONTINGENCIES - Schedule of Environmental Matters (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 site	Dec. 31, 2017 USD ($) site
Site Contingency [Line Items]
Environmental Site Quantity	60	59
The Connecticut Light And Power Company
Site Contingency [Line Items]
Environmental Site Quantity	15	14
NSTAR Electric Company
Site Contingency [Line Items]
Loss Contingency, Loss in Period | $		$ 4.9
Environmental Site Quantity	16	15
Loss Contingency, Base Rate Credit | $		$ 17.5
Environmental Site for Which a Range is Too Early to Determine
Site Contingency [Line Items]
Environmental Site Quantity	12
Environmental Site for Which a Range is Too Early to Determine | The Connecticut Light And Power Company
Site Contingency [Line Items]
Environmental Site Quantity	4
Environmental Site for Which a Range is Too Early to Determine | NSTAR Electric Company
Site Contingency [Line Items]
Environmental Site Quantity	3