RATE REDUCTION BONDS AND VARIABLE INTEREST ENTITIES - Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	$ 5,715,800	$ 6,569,900
Regulatory Liabilities	3,866,251	3,850,781
Accrued Interest (included in Other Current Liabilities)	830,620	809,214
Rate Reduction Bonds - Current Portion	43,210	43,210
Rate Reduction Bonds - Long-Term Portion	453,702	496,912
Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	478,900	522,100
Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	150,000	161,000
Special Deposits
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	78,200	73,600
Other Long-Term Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	44,700	43,600
Public Service Company of New Hampshire
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	786,400	989,100
Regulatory Liabilities	381,366	383,183
Accrued Interest (included in Other Current Liabilities)	63,005	58,487
Rate Reduction Bonds - Current Portion	43,210	43,210
Rate Reduction Bonds - Long-Term Portion	453,702	496,912
Public Service Company of New Hampshire | Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	478,900	522,100
Public Service Company of New Hampshire | Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	15,800	20,900
Public Service Company of New Hampshire | Special Deposits
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	31,400	36,800
Public Service Company of New Hampshire | Other Long-Term Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash		2,100
Variable Interest Entity, Primary Beneficiary | Public Service Company of New Hampshire
Condensed Balance Sheet Statements, Captions [Line Items]
Rate Reduction Bonds - Current Portion	43,200	43,200
Rate Reduction Bonds - Long-Term Portion	453,700	496,900
Variable Interest Entity, Primary Beneficiary | Public Service Company of New Hampshire | Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	478,900	522,100
Variable Interest Entity, Primary Beneficiary | Public Service Company of New Hampshire | Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory Liabilities	5,400	9,100
Accrued Interest (included in Other Current Liabilities)	7,500	8,000
Variable Interest Entity, Primary Beneficiary | Public Service Company of New Hampshire | Special Deposits
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	31,100	36,800
Variable Interest Entity, Primary Beneficiary | Public Service Company of New Hampshire | Other Long-Term Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	$ 3,200	$ 2,100