SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Materials, Supplies, Natural Gas, and REC Inventory (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Public Utilities, Inventory [Line Items]
Total	$ 374.4	$ 267.5
Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	221.0	148.9
Natural Gas
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	95.9	56.2
RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	57.5	62.4
CL&P
Public Utilities, Inventory [Line Items]
Total	88.2	60.3
CL&P | Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	88.2	60.3
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	138.4	116.7
NSTAR Electric | Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	81.0	55.0
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	57.4	61.7
PSNH
Public Utilities, Inventory [Line Items]
Total	34.5	25.9
PSNH | Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	34.4	25.2
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	$ 0.1	$ 0.7