COMMITMENTS AND CONTINGENCIES - Schedule of Reconciliation of Activity in Environmental Reserve (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	$ 54.9	$ 65.8
Additions	23.5	6.2
Payments/Reductions	(13.7)	(17.1)
Accrual for Environmental Loss Contingencies, Ending Balance	64.7	54.9
The Connecticut Light And Power Company
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	4.7	4.9
Additions	1.9	0.5
Payments/Reductions	(1.2)	(0.7)
Accrual for Environmental Loss Contingencies, Ending Balance	5.4	4.7
NSTAR Electric Company
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	2.7	3.8
Additions	9.7	1.8
Payments/Reductions	(1.5)	(2.9)
Accrual for Environmental Loss Contingencies, Ending Balance	10.9	2.7
Public Service Company Of New Hampshire
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	5.7	5.3
Additions	0.0	1.0
Payments/Reductions	(0.3)	(0.6)
Accrual for Environmental Loss Contingencies, Ending Balance	$ 5.4	$ 5.7