COMMITMENTS AND CONTINGENCIES - Schedule of Reconciliation of Activity in Environmental Reserve (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	$ 122.6	$ 115.4
Additions	16.8	12.6
Payments/Reductions	(11.2)	(5.4)
Accrual for Environmental Loss Contingencies, Ending Balance	$ 128.2	$ 122.6
Environmental Loss Contingency, Statement of Financial Position [Extensible Enumeration]	Other Current Liabilities	Other Current Liabilities	Other Current Liabilities
CL&P
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	$ 13.9	$ 13.9
Additions	2.6	0.9
Payments/Reductions	(2.7)	(0.9)
Accrual for Environmental Loss Contingencies, Ending Balance	13.8	13.9
NSTAR Electric
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	3.4	3.3
Additions	2.5	0.4
Payments/Reductions	(0.5)	(0.3)
Accrual for Environmental Loss Contingencies, Ending Balance	5.4	3.4
PSNH
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	6.1	6.3
Additions	1.7	0.5
Payments/Reductions	(0.2)	(0.7)
Accrual for Environmental Loss Contingencies, Ending Balance	$ 7.6	$ 6.1