ASSETS HELD FOR SALE - Generation Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Accumulated Depreciation	$ (7,390,800)	$ (7,133,800)
Net Plant	24,967,702	23,617,463
Other Assets	759,000	693,700
Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Accumulated Depreciation	(762,100)	(751,800)
Net Plant	$ 2,826,541	2,642,274
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)
Net Plant		599,000
Fuel and Inventory		87,700
Materials and Supplies		27,300
Emissions Allowances		19,100
Other Assets		2,600
Deferred Costs from Thermal Generation Asset Sale		(516,100)
Total Generation Assets Held for Sale		$ 219,600