REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 5,178,400	$ 4,664,900
Less:  Current Portion	1,064,050	632,282
Total Long-Term Regulatory Liabilities	4,114,348	4,032,564
EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,401,300	2,442,700
Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,129,200	702,400
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	764,700	684,100
Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	488,600	427,100
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	171,400	154,800
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	53,800	69,300
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	169,400	184,500
CL&P
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,831,800	1,520,000
Less:  Current Portion	391,283	124,122
Total Long-Term Regulatory Liabilities	1,440,547	1,395,883
CL&P | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	946,000	956,600
CL&P | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	443,600	180,300
CL&P | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	258,900	212,800
CL&P | Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	70,000	61,600
CL&P | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	68,900	65,100
CL&P | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	5,400	4,500
CL&P | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	39,000	39,100
NSTAR Electric
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,213,500	2,079,400
Less:  Current Portion	543,194	436,312
Total Long-Term Regulatory Liabilities	1,670,310	1,643,079
NSTAR Electric | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	858,000	877,600
NSTAR Electric | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	520,100	413,600
NSTAR Electric | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	468,400	451,300
NSTAR Electric | Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	242,500	211,600
NSTAR Electric | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	102,500	89,700
NSTAR Electric | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	8,400	21,400
NSTAR Electric | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	13,600	14,200
PSNH
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	513,400	516,100
Less:  Current Portion	103,599	121,058
Total Long-Term Regulatory Liabilities	409,842	394,982
PSNH | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	323,600	330,600
PSNH | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	97,000	114,400
PSNH | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	37,400	20,100
PSNH | Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	47,100	42,600
PSNH | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
PSNH | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	4,300	3,900
PSNH | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,000	$ 4,500