RATE REDUCTION BONDS AND VARIABLE INTEREST ENTITIES - Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Condensed Income Statements, Captions [Line Items]
Interest Expense on RRB Principal (included in Interest Expense)	$ 126,404	$ 107,329	$ 247,533	$ 210,758
Public Service Company of New Hampshire
Condensed Income Statements, Captions [Line Items]
Interest Expense on RRB Principal (included in Interest Expense)	14,612	$ 12,970	27,386	$ 25,780
Variable Interest Entity, Primary Beneficiary | Rate Reduction Bonds | Public Service Company of New Hampshire
Condensed Income Statements, Captions [Line Items]
Amortization of Regulatory Assets, Net	6,800		6,800
Interest Expense on RRB Principal (included in Interest Expense)	$ 2,800		$ 2,800