ASSETS HELD FOR SALE - Generation Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Accumulated Depreciation	$ (7,227,500)	$ (7,133,800)
Net Plant	23,958,688	23,617,463
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Accumulated Depreciation	(734,900)	(751,800)
Net Plant	2,693,951	$ 2,642,274
Electricity Generation Plant, Non-Nuclear | Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Hydroelectric Gross Plant	86,600
Accumulated Depreciation	(27,400)
Net Plant	59,200
Materials and Supplies	100
Total Generation Assets	$ 59,300