COMMITMENTS AND CONTINGENCIES - Schedule of Environmental Matters (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) site	Dec. 31, 2017 USD ($) site
Site Contingency [Line Items]
Number of Sites | site	60	59
Reserve | $	$ 66.6	$ 54.9
The Connecticut Light and Power Company
Site Contingency [Line Items]
Number of Sites | site	14	14
Reserve | $	$ 5.0	$ 4.7
NSTAR Electric Company
Site Contingency [Line Items]
Number of Sites | site	17	15
Reserve | $	$ 10.9	$ 2.7
Public Service Company of New Hampshire
Site Contingency [Line Items]
Number of Sites | site	9	10
Reserve | $	$ 5.5	$ 5.7