COMMITMENTS AND CONTINGENCIES - Schedule of Number of Sites and Reserves (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) site	Dec. 31, 2017 USD ($) site	Dec. 31, 2016 USD ($)
Loss Contingencies [Line Items]
Number of Sites | site	60	59
Reserve | $	$ 64.7	$ 54.9	$ 65.8
The Connecticut Light And Power Company
Loss Contingencies [Line Items]
Number of Sites | site	15	14
Reserve | $	$ 5.4	$ 4.7	4.9
NSTAR Electric Company
Loss Contingencies [Line Items]
Number of Sites | site	16	15
Reserve | $	$ 10.9	$ 2.7	3.8
Public Service Company Of New Hampshire
Loss Contingencies [Line Items]
Number of Sites | site	9	10
Reserve | $	$ 5.4	$ 5.7	$ 5.3