SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Summary of Accounting Policies [Line Items]
Other income, net	$ 91,030	$ 88,981
Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	27,800	34,800
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	26,800	15,500
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	7,800	3,800
Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	(800)	(1,700)
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	30,000	23,100
Other
Summary of Accounting Policies [Line Items]
Other income, net	(600)	13,500
CL&P
Summary of Accounting Policies [Line Items]
Other income, net	16,468	14,937
CL&P | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	6,500	9,500
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	6,000	4,100
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	(700)	(600)
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	4,700	1,900
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	47,114	39,873
NSTAR Electric | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	12,300	14,700
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	16,200	9,500
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	200	100
NSTAR Electric | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	400	(500)
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	18,000	16,000
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other income, net	0	100
PSNH
Summary of Accounting Policies [Line Items]
Other income, net	7,050	5,717
PSNH | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	3,600	4,200
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	1,900	700
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	(200)	(100)
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	1,800	900
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 0	$ 0