SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fuel, Materials, Supplies and Inventory (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Public Utilities, Inventory [Line Items]
Total	$ 267.5	$ 265.6
Fuel
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	56.2	38.2
Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	148.9	151.3
RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	62.4	76.1
The Connecticut Light and Power Company
Public Utilities, Inventory [Line Items]
Total	60.3	57.9
The Connecticut Light and Power Company | Fuel
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	0.0
The Connecticut Light and Power Company | Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	60.3	57.9
The Connecticut Light and Power Company | RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	0.0
NSTAR Electric Company
Public Utilities, Inventory [Line Items]
Total	116.7	133.9
NSTAR Electric Company | Fuel
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	0.0
NSTAR Electric Company | Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	55.0	62.1
NSTAR Electric Company | RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	61.7	71.8
Public Service Company of New Hampshire
Public Utilities, Inventory [Line Items]
Total	25.9	26.8
Public Service Company of New Hampshire | Fuel
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	0.0
Public Service Company of New Hampshire | Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	25.2	22.5
Public Service Company of New Hampshire | RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	$ 0.7	$ 4.3