SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Materials, Supplies, Natural Gas, and REC Inventory (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Public Utilities, Inventory [Line Items]
Total	$ 491.6	$ 594.6
Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	400.7	498.6
Natural Gas
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	49.7	49.5
RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	41.2	46.5
CL&P
Public Utilities, Inventory [Line Items]
Total	133.9	217.3
CL&P | Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	133.9	217.3
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	0.0
NSTAR Electric
Public Utilities, Inventory [Line Items]
Total	210.9	220.6
NSTAR Electric | Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	172.8	177.8
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	38.1	42.8
PSNH
Public Utilities, Inventory [Line Items]
Total	58.9	75.8
PSNH | Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	55.8	72.1
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	$ 3.1	$ 3.7