COMMITMENTS AND CONTINGENCIES - Schedule of Environmental Matters (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) site	Dec. 31, 2016 USD ($) site
Site Contingency [Line Items]
Number of Sites | site	58	61
Reserve | $	$ 57.7	$ 65.8
The Connecticut Light And Power Company
Site Contingency [Line Items]
Number of Sites | site	14	14
Reserve | $	$ 4.9	$ 4.9
NSTAR Electric Company
Site Contingency [Line Items]
Number of Sites | site	10	13
Reserve | $	$ 2.0	$ 3.2
Public Service Company Of New Hampshire
Site Contingency [Line Items]
Number of Sites | site	11	11
Reserve | $	$ 5.7	$ 5.3
Western Massachusetts Electric Company
Site Contingency [Line Items]
Number of Sites | site	4	4
Reserve | $	$ 0.8	$ 0.6