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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Materials, Supplies, Natural Gas, and REC Inventory (Details) - USD ($)
$ in Millions
Dec. 31, 2024
Dec. 31, 2023
Public Utilities, Inventory [Line Items]    
Total $ 594.6 $ 507.3
Materials and Supplies    
Public Utilities, Inventory [Line Items]    
Other inventory, capitalized costs, gross 498.6 397.9
Natural Gas    
Public Utilities, Inventory [Line Items]    
Other inventory, capitalized costs, gross 49.5 65.5
RECs    
Public Utilities, Inventory [Line Items]    
Other inventory, capitalized costs, gross 46.5 43.9
CL&P    
Public Utilities, Inventory [Line Items]    
Total 217.3 156.5
CL&P | Materials and Supplies    
Public Utilities, Inventory [Line Items]    
Other inventory, capitalized costs, gross 217.3 156.2
CL&P | Natural Gas    
Public Utilities, Inventory [Line Items]    
Other inventory, capitalized costs, gross 0.0 0.0
CL&P | RECs    
Public Utilities, Inventory [Line Items]    
Other inventory, capitalized costs, gross 0.0 0.3
NSTAR Electric    
Public Utilities, Inventory [Line Items]    
Total 220.6 173.8
NSTAR Electric | Materials and Supplies    
Public Utilities, Inventory [Line Items]    
Other inventory, capitalized costs, gross 177.8 130.8
NSTAR Electric | Natural Gas    
Public Utilities, Inventory [Line Items]    
Other inventory, capitalized costs, gross 0.0 0.0
NSTAR Electric | RECs    
Public Utilities, Inventory [Line Items]    
Other inventory, capitalized costs, gross 42.8 43.0
PSNH    
Public Utilities, Inventory [Line Items]    
Total 75.8 77.1
PSNH | Materials and Supplies    
Public Utilities, Inventory [Line Items]    
Other inventory, capitalized costs, gross 72.1 76.5
PSNH | Natural Gas    
Public Utilities, Inventory [Line Items]    
Other inventory, capitalized costs, gross 0.0 0.0
PSNH | RECs    
Public Utilities, Inventory [Line Items]    
Other inventory, capitalized costs, gross $ 3.7 $ 0.6