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FAIR VALUE OF FINANCIAL INSTRUMENTS - Fair Value Hierarchy (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Preferred Stock Not Subject to Mandatory Redemption $ 155,568 $ 155,569
Rate Reduction Bonds 324,072 367,282
Carrying Amount    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Preferred Stock Not Subject to Mandatory Redemption 155,600 155,600
Long-Term Debt 26,704,800 24,413,500
Rate Reduction Bonds 367,300 410,500
Fair Value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Preferred Stock Not Subject to Mandatory Redemption 123,800 122,200
Long-Term Debt 24,791,400 22,855,200
Rate Reduction Bonds 352,100 395,000
CL&P    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Preferred Stock Not Subject to Mandatory Redemption 116,200 116,200
CL&P | Carrying Amount    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Preferred Stock Not Subject to Mandatory Redemption 116,200 116,200
Long-Term Debt 5,111,100 4,814,400
Rate Reduction Bonds 0 0
CL&P | Fair Value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Preferred Stock Not Subject to Mandatory Redemption 90,300 90,400
Long-Term Debt 4,705,800 4,572,000
CL&P | Fair Value | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Rate Reduction Bonds 0 0
NSTAR Electric    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Preferred Stock Not Subject to Mandatory Redemption 43,000 43,000
NSTAR Electric | Carrying Amount    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Preferred Stock Not Subject to Mandatory Redemption 43,000 43,000
Long-Term Debt 5,094,900 4,496,900
Rate Reduction Bonds 0 0
NSTAR Electric | Fair Value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Preferred Stock Not Subject to Mandatory Redemption 33,500 31,800
Long-Term Debt 4,759,400 4,273,700
NSTAR Electric | Fair Value | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Rate Reduction Bonds 0 0
PSNH    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Rate Reduction Bonds 324,072 367,282
PSNH | Carrying Amount    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Preferred Stock Not Subject to Mandatory Redemption 0 0
Long-Term Debt 1,732,100 1,431,600
Rate Reduction Bonds 367,300 410,500
PSNH | Fair Value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Preferred Stock Not Subject to Mandatory Redemption 0 0
Long-Term Debt 1,529,700 1,292,600
Rate Reduction Bonds $ 352,100 $ 395,000