FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred Stock Not Subject to Mandatory Redemption	$ 155,568	$ 155,568
Rate Reduction Bonds	302,467	324,072
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	27,599,000	26,704,800
Rate Reduction Bonds	345,700	367,300
Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred Stock Not Subject to Mandatory Redemption	124,800	123,800
Long-Term Debt	26,038,900	24,791,400
Rate Reduction Bonds	336,700	352,100
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	5,508,100	5,111,100
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	92,000	90,300
Long-Term Debt	5,168,900	4,705,800
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	5,887,200	5,094,900
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	32,800	33,500
Long-Term Debt	5,617,400	4,759,400
Rate Reduction Bonds	0	0
PSNH
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Rate Reduction Bonds	302,467	324,072
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	1,732,400	1,732,100
Rate Reduction Bonds	345,700	367,300
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	1,551,600	1,529,700
Rate Reduction Bonds	$ 336,700	$ 352,100