COMMITMENTS AND CONTINGENCIES - Schedule of Number of Sites and Reserves (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) site	Dec. 31, 2015 USD ($) site	Dec. 31, 2014 USD ($)
Loss Contingencies [Line Items]
Number of Sites | site	61	64
Reserve | $	$ 65.8	$ 51.1	$ 43.3
The Connecticut Light And Power Company [Member]
Loss Contingencies [Line Items]
Number of Sites | site	14	14
Reserve | $	$ 4.9	$ 4.6	3.8
NSTAR Electric Company [Member]
Loss Contingencies [Line Items]
Number of Sites | site	13	15
Reserve | $	$ 3.2	$ 2.4	1.1
Public Service Company Of New Hampshire [Member]
Loss Contingencies [Line Items]
Number of Sites | site	11	12
Reserve | $	$ 5.3	$ 4.5	5.2
Western Massachusetts Electric Company [Member]
Loss Contingencies [Line Items]
Number of Sites | site	4	4
Reserve | $	$ 0.6	$ 0.6	$ 0.5