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COMMITMENTS AND CONTINGENCIES - Schedule of Reconciliation of Activity in Environmental Reserve (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Accrual for Environmental Loss Contingencies [Roll Forward]    
Accrual for Environmental Loss Contingencies, Beginning Balance $ 128.2 $ 122.6
Additions 11.0 16.8
Payments/Reductions (11.2) (11.2)
Accrual for Environmental Loss Contingencies, Ending Balance 128.0 $ 128.2
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 13.8 $ 13.9
Additions 0.8 2.6
Payments/Reductions (1.2) (2.7)
Accrual for Environmental Loss Contingencies, Ending Balance 13.4 13.8
NSTAR Electric    
Accrual for Environmental Loss Contingencies [Roll Forward]    
Accrual for Environmental Loss Contingencies, Beginning Balance 5.4 3.4
Additions 1.9 2.5
Payments/Reductions (0.7) (0.5)
Accrual for Environmental Loss Contingencies, Ending Balance 6.6 5.4
PSNH    
Accrual for Environmental Loss Contingencies [Roll Forward]    
Accrual for Environmental Loss Contingencies, Beginning Balance 7.6 6.1
Additions 0.2 1.7
Payments/Reductions (1.5) (0.2)
Accrual for Environmental Loss Contingencies, Ending Balance $ 6.3 $ 7.6