COMMITMENTS AND CONTINGENCIES - Schedule of Reconciliation of Activity in Environmental Reserve (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	$ 128.0	$ 128.2
Additions	40.5	11.0
Payments/Reductions	(14.2)	(11.2)
Accrual for Environmental Loss Contingencies, Ending Balance	154.3	$ 128.0
Environmental Loss Contingency, Statement of Financial Position [Extensible Enumeration]		Other Current Liabilities, Other Long-Term Liabilities
CL&P
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	13.4	$ 13.8
Additions	3.6	0.8
Payments/Reductions	(2.1)	(1.2)
Accrual for Environmental Loss Contingencies, Ending Balance	14.9	13.4
NSTAR Electric
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	6.6	5.4
Additions	1.8	1.9
Payments/Reductions	(1.4)	(0.7)
Accrual for Environmental Loss Contingencies, Ending Balance	7.0	6.6
PSNH
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	6.3	7.6
Additions	3.3	0.2
Payments/Reductions	(0.4)	(1.5)
Accrual for Environmental Loss Contingencies, Ending Balance	$ 9.2	$ 6.3