FAIR VALUE OF FINANCIAL INSTRUMENTS - Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Dec. 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	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	18,216,700	16,179,100
Rate Reduction Bonds	496,900	540,100
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred Stock Not Subject to Mandatory Redemption	166,300	169,100
Long-Term Debt	19,636,300	18,420,100
Rate Reduction Bonds	543,300	603,400
The Connecticut Light and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred Stock Not Subject to Mandatory Redemption	116,200	116,200
The Connecticut Light and Power Company | 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	4,215,400	3,914,800
The Connecticut Light and Power Company | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred Stock Not Subject to Mandatory Redemption	122,300	123,400
Long-Term Debt	4,848,900	4,800,900
NSTAR Electric Company
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 Company | 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	3,985,400	3,643,200
NSTAR Electric Company | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred Stock Not Subject to Mandatory Redemption	44,000	45,700
Long-Term Debt	4,453,500	4,294,000
Public Service Company of New Hampshire
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Rate Reduction Bonds	453,702	496,912
Public Service Company of New Hampshire | 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	1,163,800	1,099,100
Rate Reduction Bonds	496,900	540,100
Public Service Company of New Hampshire | 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	1,220,600	1,207,000
Rate Reduction Bonds	$ 543,300	$ 603,400