COMMITMENTS AND CONTINGENCIES - Schedule of Environmental Matters (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) site	Dec. 31, 2017 USD ($) site
Site Contingency [Line Items]
Number of Sites | site	58	59
Reserve | $	$ 63.8	$ 54.9
The Connecticut Light And Power Company
Site Contingency [Line Items]
Number of Sites | site	14	14
Reserve | $	$ 5.1	$ 4.7
NSTAR Electric Company
Site Contingency [Line Items]
Number of Sites | site	15	15
Reserve | $	$ 2.8	$ 2.7
Public Service Company Of New Hampshire
Site Contingency [Line Items]
Number of Sites | site	9	10
Reserve | $	$ 5.6	$ 5.7