RATE REDUCTION BONDS AND VARIABLE INTEREST ENTITIES - Income Statement (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Income Statements, Captions [Line Items]
Interest Expense on RRB Principal (included in Interest Expense)	$ 538,452	$ 533,197	$ 498,805
Public Service Company of New Hampshire
Condensed Income Statements, Captions [Line Items]
Amortization of Regulatory Assets, Net	52,804	57,732	80,978
Interest Expense on RRB Principal (included in Interest Expense)	58,127	60,666	$ 60,634
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	43,200	43,000
Interest Expense on RRB Principal (included in Interest Expense)	$ 19,700	$ 21,100