REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,692,900	$ 4,614,700
Less:  Current Portion	641,472	591,750
Total Long-Term Regulatory Liabilities	4,051,377	4,022,923
EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,537,000	2,548,600
Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	711,700	668,300
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	687,600	666,600
Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	373,200	354,000
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	135,500	124,300
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	44,800	51,000
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	203,100	201,900
CL&P
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,451,200	1,418,100
Less:  Current Portion	117,335	102,239
Total Long-Term Regulatory Liabilities	1,333,875	1,315,928
CL&P | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	966,100	969,200
CL&P | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	167,500	154,000
CL&P | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	168,200	157,900
CL&P | Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	53,100	49,900
CL&P | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	58,500	56,100
CL&P | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	500	600
CL&P | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	37,300	30,400
NSTAR Electric
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,008,300	1,953,400
Less:  Current Portion	405,190	368,070
Total Long-Term Regulatory Liabilities	1,603,079	1,585,311
NSTAR Electric | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	899,500	905,100
NSTAR Electric | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	384,200	347,200
NSTAR Electric | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	431,000	420,900
NSTAR Electric | Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	184,900	175,900
NSTAR Electric | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	77,000	68,200
NSTAR Electric | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	18,000	22,200
NSTAR Electric | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	13,700	13,900
PSNH
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	475,500	511,100
Less:  Current Portion	86,141	117,515
Total Long-Term Regulatory Liabilities	389,427	393,574
PSNH | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	337,500	339,300
PSNH | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	80,200	114,400
PSNH | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	14,800	16,200
PSNH | Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	38,200	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	$ 4,800	$ 4,600