COMMITMENTS AND CONTINGENCIES - Schedule of Number of Sites and Reserves (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) site	Dec. 31, 2016 USD ($) site	Dec. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Number of Sites | site	59	61
Reserve | $	$ 54.9	$ 65.8	$ 51.1
The Connecticut Light And Power Company
Loss Contingencies [Line Items]
Number of Sites | site	14	14
Reserve | $	$ 4.7	$ 4.9	4.6
NSTAR Electric Company
Loss Contingencies [Line Items]
Number of Sites | site	15	17
Reserve | $	$ 2.7	$ 3.8	3.0
Public Service Company Of New Hampshire
Loss Contingencies [Line Items]
Number of Sites | site	10	11
Reserve | $	$ 5.7	$ 5.3	$ 4.5