FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Thousands	Mar. 31, 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	475,307	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	15,993,900	16,179,100
Rate Reduction Bonds	518,500	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	168,300	169,100
Long-Term Debt	17,198,200	18,420,100
Rate Reduction Bonds	568,000	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	3,914,700	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	123,200	123,400
Long-Term Debt	4,423,900	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	3,643,800	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	45,100	45,700
Long-Term Debt	4,063,700	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	475,307	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	977,300	1,099,100
Rate Reduction Bonds	518,500	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	1,011,000	1,207,000
Rate Reduction Bonds	$ 568,000	$ 603,400