ASSETS HELD FOR SALE - Generation Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Accumulated Depreciation	$ (7,363,100)	$ (7,133,800)
Net Plant	24,476,890	23,617,463
Other Assets	706,400	693,700
Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Accumulated Depreciation	(754,500)	(751,800)
Net Plant	2,763,610	2,642,274
Electricity Generation Plant, Non-Nuclear | Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Thermal Gross Plant	0	1,091,400
Hydroelectric Gross Plant	86,800	83,000
Accumulated Depreciation	(27,500)	(575,400)
Net Plant	59,300	599,000
Fuel and Inventory	0	87,700
Materials and Supplies	100	27,300
Emissions Allowances	0	19,100
Other Assets	0	2,600
Deferred Costs from Thermal Generation Asset Sale	0	(516,100)
Total Generation Assets Held for Sale	$ 59,400	$ 219,600