COMMITMENTS AND CONTINGENCIES - Schedule of Environmental Matters (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) site	Dec. 31, 2016 USD ($) site
Site Contingency [Line Items]
Number of Sites | site	60	61
Reserve | $	$ 58.2	$ 65.8
The Connecticut Light And Power Company [Member]
Site Contingency [Line Items]
Number of Sites | site	14	14
Reserve | $	$ 5.3	$ 4.9
NSTAR Electric Company [Member]
Site Contingency [Line Items]
Number of Sites | site	13	13
Reserve | $	$ 1.0	$ 3.2
Public Service Company Of New Hampshire [Member]
Site Contingency [Line Items]
Number of Sites | site	11	11
Reserve | $	$ 5.3	$ 5.3
Western Massachusetts Electric Company [Member]
Site Contingency [Line Items]
Number of Sites | site	4	4
Reserve | $	$ 0.7	$ 0.6