REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,842,800	$ 4,664,900
Less:  Current Portion	774,549	632,282
Total Long-Term Regulatory Liabilities	4,068,329	4,032,564
EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,432,400	2,442,700
Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	840,500	702,400
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	711,000	684,100
Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	448,100	427,100
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	160,300	154,800
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	63,400	69,300
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	187,100	184,500
CL&P
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,581,800	1,520,000
Less:  Current Portion	169,066	124,122
Total Long-Term Regulatory Liabilities	1,412,685	1,395,883
CL&P | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	954,600	956,600
CL&P | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	221,800	180,300
CL&P | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	226,800	212,800
CL&P | Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	64,500	61,600
CL&P | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	66,600	65,100
CL&P | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	4,700	4,500
CL&P | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	42,800	39,100
NSTAR Electric
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,151,800	2,079,400
Less:  Current Portion	497,405	436,312
Total Long-Term Regulatory Liabilities	1,654,352	1,643,079
NSTAR Electric | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	872,000	877,600
NSTAR Electric | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	475,000	413,600
NSTAR Electric | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	457,600	451,300
NSTAR Electric | Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	222,300	211,600
NSTAR Electric | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	93,700	89,700
NSTAR Electric | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	17,100	21,400
NSTAR Electric | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	14,100	14,200
PSNH
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	504,000	516,100
Less:  Current Portion	103,097	121,058
Total Long-Term Regulatory Liabilities	400,903	394,982
PSNH | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	328,600	330,600
PSNH | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	96,100	114,400
PSNH | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	26,600	20,100
PSNH | Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	44,100	42,600
PSNH | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
PSNH | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	4,000	3,900
PSNH | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,600	$ 4,500