SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Materials, Supplies, Natural Gas, and REC Inventory (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Public Utilities, Inventory [Line Items]
Total	$ 507.3	$ 374.4
Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	397.9	221.0
Natural Gas
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	65.5	95.9
RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	43.9	57.5
CL&P
Public Utilities, Inventory [Line Items]
Total	156.5	88.2
CL&P | Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	156.2	88.2
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	0.3	0.0
NSTAR Electric
Public Utilities, Inventory [Line Items]
Total	173.8	138.4
NSTAR Electric | Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	130.8	81.0
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	43.0	57.4
PSNH
Public Utilities, Inventory [Line Items]
Total	77.1	34.5
PSNH | Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	76.5	34.4
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	$ 0.6	$ 0.1