COMMITMENTS AND CONTINGENCIES - Schedule of Reconciliation of Activity in Environmental Reserve (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	$ 65.8	$ 51.1
Additions	6.2	20.6
Payments/Reductions	(17.1)	(5.9)
Accrual for Environmental Loss Contingencies, Ending Balance	54.9	65.8
The Connecticut Light And Power Company
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	4.9	4.6
Additions	0.5	0.6
Payments/Reductions	(0.7)	(0.3)
Accrual for Environmental Loss Contingencies, Ending Balance	4.7	4.9
NSTAR Electric Company
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	3.8	3.0
Additions	1.8	1.8
Payments/Reductions	(2.9)	(1.0)
Accrual for Environmental Loss Contingencies, Ending Balance	2.7	3.8
Public Service Company Of New Hampshire
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	5.3	4.5
Additions	1.0	1.2
Payments/Reductions	(0.6)	(0.4)
Accrual for Environmental Loss Contingencies, Ending Balance	$ 5.7	$ 5.3