COMMITMENTS AND CONTINGENCIES - Schedule of Reconciliation of Activity in Environmental Reserve (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	$ 64.7	$ 54.9
Additions	26.5	23.5
Payments/Reductions	(10.2)	(13.7)
Accrual for Environmental Loss Contingencies, Ending Balance	81.0	64.7
CL&P
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	5.4	4.7
Additions	7.0	1.9
Payments/Reductions	(1.0)	(1.2)
Accrual for Environmental Loss Contingencies, Ending Balance	11.4	5.4
NSTAR Electric
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	10.9	2.7
Additions	0.5	9.7
Payments/Reductions	(3.4)	(1.5)
Accrual for Environmental Loss Contingencies, Ending Balance	8.0	10.9
PSNH
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	5.4	5.7
Additions	2.8	0.0
Payments/Reductions	(0.7)	(0.3)
Accrual for Environmental Loss Contingencies, Ending Balance	$ 7.5	$ 5.4