SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fuel, Materials, Supplies and Inventory (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Public Utilities, Inventory [Line Items]
Total - Current	$ 238.0	$ 223.1
Fuel
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	33.1	29.7
Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	126.1	117.1
RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	78.8	76.3
The Connecticut Light And Power Company
Public Utilities, Inventory [Line Items]
Total - Current	44.5	48.4
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	44.5	44.4
The Connecticut Light And Power Company | RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	4.0
NSTAR Electric Company
Public Utilities, Inventory [Line Items]
Total - Current	114.2	95.5
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	48.6	45.1
NSTAR Electric Company | RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	65.6	50.4
Public Service Company Of New Hampshire
Public Utilities, Inventory [Line Items]
Total - Current	37.5	40.3
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	24.3	18.5
Public Service Company Of New Hampshire | RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	$ 13.2	$ 21.8