COMMITMENTS AND CONTINGENCIES - Schedule of Environmental Matters (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($) site	Dec. 31, 2015 USD ($) site
Site Contingency [Line Items]
Number of Sites | site	63	64
Reserve | $	$ 59.7	$ 51.1
The Connecticut Light And Power Company [Member]
Site Contingency [Line Items]
Number of Sites | site	14	14
Reserve | $	$ 4.7	$ 4.6
NSTAR Electric Company [Member]
Site Contingency [Line Items]
Number of Sites | site	14	15
Reserve | $	$ 2.3	$ 2.4
Public Service Company Of New Hampshire [Member]
Site Contingency [Line Items]
Number of Sites | site	12	12
Reserve | $	$ 4.3	$ 4.5
Western Massachusetts Electric Company [Member]
Site Contingency [Line Items]
Number of Sites | site	4	4
Reserve | $	$ 0.7	$ 0.6