FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
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	432,097	453,702
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	18,729,600	18,216,700
Rate Reduction Bonds	475,300	496,900
Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred Stock Not Subject to Mandatory Redemption	158,300	166,300
Long-Term Debt	18,562,300	19,636,300
Rate Reduction Bonds	492,800	543,300
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	4,215,700	4,215,400
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	115,500	122,300
Long-Term Debt	4,392,200	4,848,900
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	3,986,300	3,985,400
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	42,800	44,000
Long-Term Debt	4,094,100	4,453,500
Rate Reduction Bonds	0	0
PSNH
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Rate Reduction Bonds	432,097	453,702
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	1,164,000	1,163,800
Rate Reduction Bonds	475,300	496,900
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	1,086,300	1,220,600
Rate Reduction Bonds	$ 492,800	$ 543,300