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FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Preferred Stock Not Subject to Mandatory Redemption | $ 155,570 | $ 155,570
Rate Reduction Bonds | 453,702 | 496,912
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 | 18,219,200 | 16,179,100
Rate Reduction Bonds | 496,900 | 540,100
Fair Value | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Preferred Stock Not Subject to Mandatory Redemption | 165,500 | 169,100
Long-Term Debt | 19,810,900 | 18,420,100
Rate Reduction Bonds | 555,800 | 603,400
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 | 4,215,100 | 3,914,800
Rate Reduction Bonds | 0 | 0
CL&P | Fair Value | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Preferred Stock Not Subject to Mandatory Redemption | 121,400 | 123,400
Long-Term Debt | 4,865,200 | 4,800,900
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 | 3,984,700 | 3,643,200
Rate Reduction Bonds | 0 | 0
NSTAR Electric | Fair Value | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Preferred Stock Not Subject to Mandatory Redemption | 44,100 | 45,700
Long-Term Debt | 4,494,300 | 4,294,000
Rate Reduction Bonds | 0 | 0
PSNH |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Rate Reduction Bonds | 453,702 | 496,912
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 | 1,163,600 | 1,099,100
Rate Reduction Bonds | 496,900 | 540,100
PSNH | Fair Value | Level 2 |  |
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 | 1,228,900 | 1,207,000
Rate Reduction Bonds | $ 555,800 | $ 603,400

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