SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fuel, Materials, Supplies and Inventory (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Public Utilities, Inventory [Line Items]
Total	$ 235.5	$ 238.0
Fuel
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	26.7	33.1
Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	132.9	126.1
RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	75.9	78.8
CL&P
Public Utilities, Inventory [Line Items]
Total	50.7	44.5
CL&P | Fuel
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	0.0
CL&P | Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	50.7	44.5
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	124.1	114.2
NSTAR Electric | Fuel
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	0.0
NSTAR Electric | Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	54.7	48.6
NSTAR Electric | RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	69.4	65.6
PSNH
Public Utilities, Inventory [Line Items]
Total	25.0	37.5
PSNH | Fuel
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	0.0
PSNH | Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	18.5	24.3
PSNH | RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	$ 6.5	$ 13.2