SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Summary of Accounting Policies [Line Items]
Other income, net	$ 92,344	$ 91,030
Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	32,600	27,800
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	23,900	26,800
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	6,000	7,800
Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	(2,000)	(800)
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	34,000	30,000
Other
Summary of Accounting Policies [Line Items]
Other income, net	(2,200)	(600)
CL&P
Summary of Accounting Policies [Line Items]
Other income, net	17,599	16,468
CL&P | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	8,300	6,500
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	3,600	6,000
CL&P | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	0	0
CL&P | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	100	(700)
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	5,600	4,700
CL&P | Other
Summary of Accounting Policies [Line Items]
Other income, net	0	0
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other income, net	46,292	47,114
NSTAR Electric | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	14,800	12,300
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	14,100	16,200
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	200	200
NSTAR Electric | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	(1,800)	400
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	19,000	18,000
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other income, net	0	0
PSNH
Summary of Accounting Policies [Line Items]
Other income, net	10,758	7,050
PSNH | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	4,100	3,600
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	3,900	1,900
PSNH | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	0	0
PSNH | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	100	(200)
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	2,700	1,800
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 0	$ 0