SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fuel, Materials, Supplies and Inventory (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fuel [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	$ 135.7	$ 152.5
Materials and Supplies [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	142.7	131.2
RECs [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	47.9	50.9
Emission Allowances [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	2.4	1.9
Other inventory, noncurrent	17.5	17.5
The Connecticut Light And Power Company [Member] | Fuel [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	0.0
The Connecticut Light And Power Company [Member] | Materials and Supplies [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	48.2	43.1
The Connecticut Light And Power Company [Member] | RECs [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	3.9	0.0
The Connecticut Light And Power Company [Member] | Emission Allowances [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	0.0
Other inventory, noncurrent	0.0	0.0
NSTAR Electric Company [Member] | Fuel [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	0.0
NSTAR Electric Company [Member] | Materials and Supplies [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	34.5	32.2
NSTAR Electric Company [Member] | RECs [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	27.8	43.3
NSTAR Electric Company [Member] | Emission Allowances [Member]
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 [Member] | Fuel [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	99.9	103.4
Public Service Company Of New Hampshire [Member] | Materials and Supplies [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	47.3	44.6
Public Service Company Of New Hampshire [Member] | RECs [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	12.8	7.0
Public Service Company Of New Hampshire [Member] | Emission Allowances [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	2.4	1.9
Other inventory, noncurrent	17.5	17.5
Western Massachusetts Electric Company [Member] | Fuel [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	0.0
Western Massachusetts Electric Company [Member] | Materials and Supplies [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	5.2	5.4
Western Massachusetts Electric Company [Member] | RECs [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	0.6
Western Massachusetts Electric Company [Member] | Emission Allowances [Member]
Public Utilities, Inventory [Line Items]
Other inventory, capitalized costs, gross	0.0	0.0
Other inventory, noncurrent	$ 0.0	$ 0.0