RATE REDUCTION BONDS AND VARIABLE INTEREST ENTITIES - Summary of the Impact of Funding on the Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	$ 6,475,200	$ 6,569,900
Regulatory Liabilities	3,907,582	3,850,781
Accrued Interest (included in Other Current Liabilities)	802,831	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)	489,700	522,100
Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	146,900	161,000
Current Derivative Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	55,500	73,600
Other Long-Term Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	44,700	43,600
PSNH
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	927,700	989,100
Regulatory Liabilities	389,426	383,183
Accrued Interest (included in Other Current Liabilities)	61,883	58,487
Rate Reduction Bonds - Current Portion	43,210	43,210
Rate Reduction Bonds - Long-Term Portion	453,702	496,912
PSNH | VIE
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
PSNH | Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	489,700	522,100
PSNH | Securitized Stranded Costs | VIE
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	489,700	522,100
PSNH | Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	16,900	20,900
PSNH | Other Regulatory Assets | VIE
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory Liabilities	8,000	9,100
Accrued Interest (included in Other Current Liabilities)	3,000	8,000
PSNH | Current Derivative Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	18,600	36,800
PSNH | Current Derivative Assets | VIE
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	18,300	36,800
PSNH | Other Long-Term Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	3,200	2,100
PSNH | Other Long-Term Assets | VIE
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	$ 3,200	$ 2,100