SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fuel, Materials, Supplies and Inventory (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fuel
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	$ 29.7	$ 135.7
Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	117.1	142.7
RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	76.3	47.9
Emission Allowances
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	2.4
Other inventory, noncurrent	0.0	17.5
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	44.4	48.2
The Connecticut Light And Power Company | RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	4.0	3.9
The Connecticut Light And Power Company | Emission Allowances
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	0.0
Other inventory, noncurrent	0.0	0.0
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	45.1	39.7
NSTAR Electric Company | RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	50.4	31.2
NSTAR Electric Company | Emission Allowances
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	0.0
Other inventory, noncurrent	0.0	0.0
Public Service Company Of New Hampshire | Fuel
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	99.9
Public Service Company Of New Hampshire | Materials and Supplies
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	18.5	47.3
Public Service Company Of New Hampshire | RECs
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	21.8	12.8
Public Service Company Of New Hampshire | Emission Allowances
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	2.4
Other inventory, noncurrent	$ 0.0	$ 17.5