REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,847,300	$ 4,614,700
Less:  Current Portion	722,495	591,750
Total Long-Term Regulatory Liabilities	4,124,761	4,022,923
EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,512,200	2,548,600
Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	789,800	668,300
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	740,400	666,600
Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	409,300	354,000
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	148,400	124,300
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	34,900	51,000
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	212,300	201,900
CL&P
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,509,800	1,418,100
Less:  Current Portion	139,455	102,239
Total Long-Term Regulatory Liabilities	1,370,302	1,315,928
CL&P | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	960,100	969,200
CL&P | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	188,200	154,000
CL&P | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	197,500	157,900
CL&P | Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	58,800	49,900
CL&P | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	63,200	56,100
CL&P | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	400	600
CL&P | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	41,600	30,400
NSTAR Electric
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,092,600	1,953,400
Less:  Current Portion	468,756	368,070
Total Long-Term Regulatory Liabilities	1,623,764	1,585,311
NSTAR Electric | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	885,200	905,100
NSTAR Electric | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	446,900	347,200
NSTAR Electric | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	448,100	420,900
NSTAR Electric | Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	202,800	175,900
NSTAR Electric | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	85,200	68,200
NSTAR Electric | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	10,400	22,200
NSTAR Electric | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	14,000	13,900
PSNH
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	500,600	511,100
Less:  Current Portion	106,761	117,515
Total Long-Term Regulatory Liabilities	393,847	393,574
PSNH | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	333,700	339,300
PSNH | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	99,800	114,400
PSNH | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	21,100	16,200
PSNH | Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	41,100	36,600
PSNH | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
PSNH | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
PSNH | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,900	$ 4,600