REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 6,670,100	$ 7,070,700
Less:  Current Portion	1,831,224	2,189,660
Total Long-Term Regulatory Assets	4,838,923	4,880,974
Storm Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	1,955,400	2,039,400
Regulatory Tracking Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	1,495,600	1,781,600
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	1,001,300	968,400
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	976,100	967,400
Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	316,900	349,300
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	234,600	247,200
Cost of Removal
Regulatory Assets [Line Items]
Total Regulatory Assets	238,900	198,400
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	159,600	150,200
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	10,700	57,200
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	281,000	311,600
CL&P
Regulatory Assets [Line Items]
Total Regulatory Assets	1,947,500	2,325,500
Less:  Current Portion	268,821	638,529
Total Long-Term Regulatory Assets	1,678,692	1,687,029
CL&P | Storm Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	970,900	971,100
CL&P | Regulatory Tracking Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	202,200	507,700
CL&P | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	532,900	521,000
CL&P | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	167,800	168,800
CL&P | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
CL&P | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
CL&P | Cost of Removal
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
CL&P | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	43,400	41,200
CL&P | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	10,700	57,200
CL&P | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	19,600	58,500
NSTAR Electric
Regulatory Assets [Line Items]
Total Regulatory Assets	2,029,400	2,107,100
Less:  Current Portion	912,343	902,770
Total Long-Term Regulatory Assets	1,117,130	1,204,337
NSTAR Electric | Storm Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	520,900	609,800
NSTAR Electric | Regulatory Tracking Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	662,900	650,000
NSTAR Electric | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	157,700	145,400
NSTAR Electric | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	300,700	293,600
NSTAR Electric | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	201,400	212,300
NSTAR Electric | Cost of Removal
Regulatory Assets [Line Items]
Total Regulatory Assets	8,200	8,500
NSTAR Electric | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	83,000	78,300
NSTAR Electric | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	94,600	109,200
PSNH
Regulatory Assets [Line Items]
Total Regulatory Assets	984,000	1,065,700
Less:  Current Portion	118,896	173,267
Total Long-Term Regulatory Assets	865,073	892,411
PSNH | Storm Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	463,600	458,500
PSNH | Regulatory Tracking Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	110,500	162,800
PSNH | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	20,200	20,700
PSNH | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	64,300	65,600
PSNH | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	316,900	349,300
PSNH | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
PSNH | Cost of Removal
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
PSNH | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	5,400	5,100
PSNH | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
PSNH | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 3,100	$ 3,700