REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 6,063,600	$ 5,538,100
Less:  Current Portion	1,739,987	1,264,609
Total Long-Term Regulatory Liabilities	4,323,619	4,273,465
EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,407,800	2,423,400
Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,578,200	1,186,600
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	917,700	867,100
Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	534,500	509,400
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	187,300	179,400
Derivative Contract
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	131,200	91,000
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	74,900	80,400
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	232,000	200,800
CL&P
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,104,300	1,862,600
Less:  Current Portion	635,307	417,498
Total Long-Term Regulatory Liabilities	1,468,970	1,445,140
CL&P | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	928,600	933,300
CL&P | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	661,800	457,300
CL&P | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	297,500	275,100
CL&P | Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	76,200	72,800
CL&P | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	74,700	72,000
CL&P | Derivative Contract
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
CL&P | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	9,100	8,800
CL&P | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	56,400	43,300
NSTAR Electric
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,557,800	2,352,900
Less:  Current Portion	835,408	650,813
Total Long-Term Regulatory Liabilities	1,722,355	1,702,059
NSTAR Electric | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	841,200	847,900
NSTAR Electric | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	648,200	535,200
NSTAR Electric | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	498,000	479,400
NSTAR Electric | Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	264,700	252,500
NSTAR Electric | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	112,600	107,400
NSTAR Electric | Derivative Contract
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	131,200	91,000
NSTAR Electric | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	20,200	24,500
NSTAR Electric | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	41,700	15,000
PSNH
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	559,200	535,800
Less:  Current Portion	134,115	118,443
Total Long-Term Regulatory Liabilities	425,128	417,442
PSNH | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	317,300	319,800
PSNH | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	125,500	111,200
PSNH | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	54,600	45,500
PSNH | Deferred Portion of Non-Service Income Components of Pension, SERP and PBOP
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	50,900	48,500
PSNH | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
PSNH | Derivative Contract
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
PSNH | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	6,900	6,800
PSNH | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,000	$ 4,000