ASSETS HELD FOR SALE - Schedule of Assets-Held-for-Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Thermal Gross Plant	$ 29,213,900	$ 27,060,500
Accumulated Depreciation	(7,133,800)	(6,722,400)
Total Property, Plant and Equipment, Net	23,617,463	21,350,510
Other Assets	693,700	591,600
Regulatory assets	(5,239,300)	(4,526,300)
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Thermal Gross Plant	3,244,700	4,197,300
Accumulated Depreciation	(751,800)	(1,254,700)
Total Property, Plant and Equipment, Net	2,642,274	3,039,313
Regulatory assets	(940,800)	$ (362,700)
Electricity Generation Plant, Non-Nuclear | Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Thermal Gross Plant	1,091,400
Hydroelectric Gross Plant	83,000
Accumulated Depreciation	(575,400)
Total Property, Plant and Equipment, Net	599,000
Fuel and Inventory	87,700
Materials and Supplies	27,300
Emission Allowances	19,100
Other Assets	2,600
Deferred Costs from Generation Asset Sale	(516,100)
Total Generation Assets Held for Sale	$ 219,600