COMMITMENTS AND CONTINGENCIES - Schedule of Reconciliation of Activity in Environmental Reserve (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	$ 51.1	$ 43.3
Additions	20.6	13.5
Payments/Reductions	(5.9)	(5.7)
Accrual for Environmental Loss Contingencies, Ending Balance	65.8	51.1
The Connecticut Light And Power Company [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	4.6	3.8
Additions	0.6	1.3
Payments/Reductions	(0.3)	(0.5)
Accrual for Environmental Loss Contingencies, Ending Balance	4.9	4.6
NSTAR Electric Company [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	2.4	1.1
Additions	1.7	2.0
Payments/Reductions	(0.9)	(0.7)
Accrual for Environmental Loss Contingencies, Ending Balance	3.2	2.4
Public Service Company Of New Hampshire [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	4.5	5.2
Additions	1.2	2.3
Payments/Reductions	(0.4)	(3.0)
Accrual for Environmental Loss Contingencies, Ending Balance	5.3	4.5
Western Massachusetts Electric Company [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	0.6	0.5
Additions	0.1	0.2
Payments/Reductions	(0.1)	(0.1)
Accrual for Environmental Loss Contingencies, Ending Balance	$ 0.6	$ 0.6